Using a straw when drinking sugary/ acidic drinks is supposed to be better for your teeth since it limits the exposure of sugar or acid to your teeth. Not sure how true that is, but I have had dentists suggest it as a way of lowering the chance of getting cavities.

Everyone replying to you so far seems to be forgetting how you can just put a tiny hole on one edge of the lid to drink through without big ice chunks coming through

Many fast food cups lack the structural integrity to hold the quantity of liquid that they're supposed to without a lid, and using a lid necessitates using a straw

When your drink cup has a bunch of ice, it's easier to drink from with a straw.

One extremely far-fetched reason for using straws I've heard is that it's somehow more hygienic than putting your mouth on restaurant cups.

Some people have disabilities

Some people can't hold objects with any dexterity due to disabilities etc. So they use straws.

Sure, you could, absolutely. There are times when it is convenient to have one. For example, most fast food establishments will provide you a drink cup with a lid and straw. You could just rip the lid off and drink it normally, but if you are driving (which you probably are if you are getting fast food) and want a sip of your drink, it's a lot easier to just grab the cup and take a sip through a straw than it is to grab the open container and tilt it. Not to mention the chance you might hit a bump or brake hard and have it spill, either while you are trying to drink from it or even just when it's sitting in the cupholder if partially full.