There's 3 reasons actually:
1) Because when I get > an ounce off my next grow (Nirvana seeds bubblicious auto), I don't want to be smoking it for the next year. At the rate I smoke it'll take me that long, and I'd love to be able to get a bunch of different strains from a single grow without having to grow 5 plants and get busted for supply.
2) Because IMO weed shouldn't be criminalised - and it would make it super easy for medical patients to share and get hold of some medication. Where I'm from it's very difficult to do currently.
3) Because I hate the fact that the easiest place to get weed from is gangs. I hate supporting crime, and this system would mean that in any one city where there were a group of active growers there should/could be weed available to everyone.
4) Because it would encourage other people to grow their own, so they could get in on the system and be part of the swap culture. Thereby increasing the community - hopefully leading to a wider acceptance.
5) Because weed is a weed and it grows in abundance, and the system wouldn't work with any other drug so my conscience is clear building it. "Oh look - my yield off this plant is 80g in 6 weeks...". No one would give away 5g of coke so they could start swapping 5g of coke.
6) For love