Being small, inconspicious, and fast enough in his rat form, Peter would be able to slip under The Whomping Willow's frenzied dance, hit the kill switch, thus allowing James and Sirius to pass through without getting their heads smashed.
‘They couldn’t keep me company as humans, so they kept me company as animals,’ said Lupin. ‘A werewolf is only a danger to people. They sneaked out of the castle every month under James’s Invisibility Cloak. They transformed, Peter, as the smallest, could slip beneath the Willow’s attacking branches and touch the knot that freezes it. They would then slip down the tunnel and join me.
Harry Potter and the Prisoner of Azkaban
He would provide psychological support as well to Remus during his transformations.
‘And they didn’t desert me at all. Instead they did something for me that would make my transformations not only bearable, but the best times of my life. They became Animagi.’
Harry Potter and the Prisoner of Azkaban
Being "animals" in a sense, Lupin was not prone to attacking the Marauders, as such they were able to accompany him during his transformations.
How did he plan on helping before becoming an Animagus?
Uncertain, as we know that you can't choose your Animagus form (see @Gallifreyan's answer). This is pure speculation, but it's likely he intended to provide psychological support regardless.
A werewolf is generally known for not being a threat to animals, according to Remus, at least. Knowing this, the Marauders become Animagi for Remus' sake, as they knew they would not be in danger in their animal forms, thus giving him psychological support during his transformations. So whatever animal Peter would have become he would still have been able to accompany Remus during his transformations.