What do I really lose if I backup my iPhone to iTunes without transferring purchases? I don't want to waste tens of gigabytes of space on my laptop to store apps that I can just as easily download from the App Store.

Assuming I would have to recover an iPhone backup from iTunes to a brand new replacement iPhone, what will I lose if I don't agree to transfer purchases when making the backup?
I understand the apprehension!

In reality, though, you do not stand to lose anything. The only reason to transfer those purchases to your computer would be to save bandwidth and time by not re-downloading everything. You will still lose associated app data (e.g. login information, cache, etc.), but that would happen even if you did back up those apps to your computer.

In short, you will be fine without transferring purchases.

Hope this helps!