If you have poor internet, then you may prefer physical cartridges, to avoid needing to download large files.
If you are like my parents, you might not trust the internet for purchases and only buy things in physical stores - so the physical copy is better.
If you are paranoid, you might not trust Nintendo to protect your account from hackers, who might mess with your digital purchase history.
And one day will Nintendo closes it's digital store, and downloads will not be possible (has happened for DS).
For both these reasons you might get the physical copy as a guarantee that you will always have access to the game you paid for.
You may have friends.
In this case, a physical cartridge will let you lend games you've finished to your friends.
Hopefully they'll let you borrow some that they've finished.
Otherwise, yes, the convenience of digital makes that purchase method better for most people.
(edited to include suggestions from comments below)
gaming.stackexchange.com/users/169790/Sir Adelaide
Two factors why sometimes physical is better: As Trent already said, you can easily share it between friends or resell them if you don't like it.
Nintendo doesn't currently have any way to refund your purchase ( source ) The limited space on the Switch: If you only buy the online versions you might run into space problems.
The Switch currently has only 32GB of internal storage, so you might need an additional SD-Card.
gaming.stackexchange.com/users/83179/Vulcano
Nintendo offer a loyalty points reward scheme called My Nintendo for both digital and physical game purchases.
Currently the points can be used for discounts on titles from the Nintendo eshop.
Buying digital can earn up to 5x more points then a physical purchase.
For example if you bought Zelda:
BOTW Physical = 16 gold My Nintendo coins Digital =
80 gold My Nintendo coins
gaming.stackexchange.com/users/108687/PKMNCAPO 
One thing that I haven't seen mentioned yet is that for someone who wants to proudly display their game collection, physical copies are easier to display, and may additionally come with a special display item like a statue if buying a more expensive edition.
gaming.stackexchange.com/users/69166/Nzall
Physical games are better if you don't want to have to deal with Micro SD cards and USB flash drives.
However, they can be lost or damaged.
Physical games can be used on multiple consoles and can be sold.
Digital games on the Switch can be redownloaded as they are linked to your account and are more portable in the sense you don't have to carry a bunch of cartridges around.
However, you will have to wait around for long and large downloads and if you have lower internet speeds it could take hours to download.
Both types of games store the save data on the console (as opposed to the 3DS writing to cartridges), and there is currently no way to transfer it so there still is a risk if something happens to your console.
gaming.stackexchange.com/users/134045/Alex Batts
Besides factors already mentioned, like protecting yourself from the situation when official shop servers are not maintained anymore, there is also a theory that Internal storage is a bit faster than SD, which is a bit faster than cartrige.
See this video for reference.
They tested a game on various media, and although the test might not be extensive and conclusive, the fact that they constantly have a similar results might suggest that internal memory is indeed slightly faster.
gaming.stackexchange.com/users/122302/MatthewRock
First off i should make clear i don't have a Switch myself to test this with
but i read an interesting article which may influence the choice between physical purchases or digital because it may have an effect on loadtimes.
From polygon
In case physical games are read from "disc" like Nintendo used to do traditionally rather than being installed to the system's internal memory (NOT SD) and
read from there you might experience longer loadtimes.
gaming.stackexchange.com/users/125665/Justin
There are currently two options for getting new games:   Buy a physical copy   Buy a digital one   It seems that the pricing is the the same, so to me it seems that the digital one is better; you don't have to bother with a physical copy, needing to swap cartridges, and risking losing the game.  I am probably forgetting some key elements here, so is there something I overlooked?
What are the pros and cons of digital vs physical games for the Nintendo Switch?
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gaming.stackexchange.com/questions/303587
gaming.stackexchange.com/users/182273/Antzi
If you have poor internet, then you may prefer physical cartridges, to avoid needing to download large files. Physical copies are easier to display, can be used on multiple consoles and can be sold.  However, they can be lost or damaged and buying digital can earn up to 5x more points then a physical purchase.    There are a multitude of advantages for owning the phisical copy, it gets around poor internet issues, you can share the games with friends or sell them on, you can display them at home and many more. The advantages of a digital copy are that the games are more portable, you might lose or damage the cartriges and the digital copies sometimes enjoy extra game perks.
They tested a game on various media, and although the test might not be extensive and conclusive, the fact that they constantly have a similar results might suggest that internal memory is indeed slightly faster.
Most users seem to support physical copies of the game There are however some advantages to digital copies.