You will find the yeast drops out pretty good by itself in bottles, although I do use isinglass to speed it up because it's slower in a keg of course. However, I use it in the fermenter while it's cold crashing so I can leave it behind when it's kegged and I get hardly any sediment in them. Clear pours from the get go. The last pour when it blows dry sucks up a bit of yeast if I let it splutter too long but otherwise clear from start to finish.
Isinglass drops yeast just as effectively as gelatine, but rather than resulting in a jelly like sediment, it forms a very compact sediment.
Other haze could be chill haze. I use Polyclar to remove it, once again in the fermenter to leave it behind. I usually add it a couple of days after the isinglass. It works extremely well and to my taste doesn't negatively affect the flavour.
Alternatively you could use the biofine mentioned earlier which drops both yeast and chill haze causing things, and also doesn't result in a jelly sediment.
As for why some use these things, some of us like clear beer. It's no more wrong or right than not wanting it clear, just personal preference. The point is that there are much better products than gelatine to achieve it.