The store will be a place that you can see what is available to buy. Some items may have a restriction based upon your shinobi rank. The stock may rotate week to week so swing by to see what is available.
Some items, such as weapons, will not increase stats but will still be used to see who would win. ex a kunai vs a katana, majority of the time the katana is going to be better to have. So there is value in buying stuff that doesn't always have an "obvious" buff