why do people convert these? what is the purpose? i know it looks more like a sporter version of an ak and some people want to make it more military but other than that what are the benifits?
Even if you stay with a fixed stock, doing the conversion shortens up the OAL of the Rifle by a good 4". Prior to the conversion, my Saiga with its 16" barrel was roughly the same OAL as my dad's AR-15 with a 20" barrel. Even with the Warsaw-length fixed stock that I put on mine, a converted Saiga is, IMHO, a much shorter and handier rifle than an unconverted one (a folding stock would make it shorter and handier still).
Also, the trigger is improved by a conversion with a Tapco G2 trigger group. The long linkage system for the uncoverted Saiga's trigger makes it a bit gritty and spongy. While certainly no benchrest rifle, a converted Saiga's trigger is at least a good deal smoother than an unconverted on (feels much like a light revolver trigger).
Finally, you have the issue of high capacity magazines. While there are hi-caps made to work in unconverted Saigas (the SGM Tactical magazines work very well, but I can't comment on ProMags as I've never used them), the use of hi-caps in an unconverted Saiga is a legal grey area. While there is nothing in the text of Sec. 922r or 925(d)3 that specifically prohibits the use of high-capacity magazines in an unmodified imported rifle, the Chinese Mak 90's were banned from importation due to their ability to accept standard AK mags without modification to the rifle or magazine and the Saigas are specifically designed not to function with regular AK mags. While it's unlikely that you'd get in trouble for using SGM or ProMag magazines in an unconverted Saiga (I've never even heard of anyone being prosecuted under 922r, much less for a violation in such a grey area) many people feel that it is still safer, legally speaking, to only use high-capacity magazines in a converted Saiga.