Why play Land Grant?
The card has a lot of uses, similarly to fetch lands and cantrip spells. Generally:


It lets you play less actual lands (like in 1-land Belcher Decks).
It lets you find the right land (when you are using dual lands).
It's a free way to shuffle your deck.
It's a free way to increase storm count.
It filters your deck making it slightly smaller. By using free cantrips
like Land Grant and Gitaxian Probe you make your deck virtually
smaller, thus increasing the chances of all the other cards showing up 
during a match.


In this particular case, knowing how infect decks work in Pauper, I would say it's basically reason #5 (deck thinning).