one is surround sound while the other is stereo...
some of the 5.1 kits are the same speakers as the 2.1 but with more of them...
not all sources are recorded in 5.1. many streaming anime sources (crunchyroll, netflix) are stereo while even many anime movies are stereo. many regular movies are 5.1 as well as games. music is generally stereo as well as youtube, windows and many other sounds.
for the same price you can buy a higher quality 2.1 or 2.0 set than a 5.1
while pc speakers can get the job done for cheap theres cetainly a quality level that is not comparable to decent home theater options. of course the prices reflect this.
a set of stereo bookshelf speakers from the hifi market will generally be better quality audio than a 5.1 pc speaker set. the price may be a bit more but it could be worth it depending on taste.
honestly the only person who can make the call between what kind of system to get on what budget is you. if you are not picky you could certainly go with a cheap set. heck, you could even go with a cheap soundbar