The biggest land based vehicles are bucket wheel excavators. They are used to dig out vast quantities of dirt. If you look up a bucket wheel excavator (e.g. bagger 293), they kind of already look like gigantic tanks. There are a few reasons that you might want to have an ultra heavy tank which is similar to a bucket wheel excavator.
Excavation. For some reason, your military goal involves digging up a lot of dirt. This might be because you need to dig a dirt highway for logistics reasons, you need to create dirt airfields a lot, or you simply want the dirt - wars are fought over minerals and natural resources often. 658
If you're trying to extract resources from a technologically inferior enemy nation, one way to do it is to build a titanic mining machine, cover it in armour so they can't effectively harm it with small arms fire and weapons so they can't get close enough to board it, roll up, take their ore deposits, and leave.
Moving really heavy things. Suppose you have a supergun. It can sink battleships from hundreds of miles away. It's a tactically valuable tool, but you only have one (or a few) of them. You want to be able to relocate your supergun for strategic purposes, and the normal way to do that would be to disassemble it, throw it on a fleet of trucks, and ship it. Problem is, disassembling something as big as a supergun takes a day at least, so does reassembling it, and transporting isn't speedy either. If you take your supergun offline for days and the enemy finds out, you're doomed! So instead, you make it mobile. Build a bucket wheel excavator looking thing, replace the mining machinery with your supergun, and now it can move a few miles a day (faster if that's a design priority) and still be 10 minutes away from firing at all times.
If you can keep your supergun moving at 10 miles a day, and if everything in a 100 mile radius of your supergun is captured territory, you can leave from Warsaw and capture Moscow in 2 months. 898