The only use you can make of credits is that You can use those credits to chose your trip from tripoto customised trips for cheaper price. for example if they have put a trip to kasol for 10000 , using credits it will cost you less. maybe 3 to 4 k less.
You get credits for posting trips and/or photos and videos. You also get credits for answering questions. Everytime you earn credits, you get an email about it (atleast for credits earned for posting a trip/photo)