The phrase "review purposes" means to examine or assess something for a specific reason, often to gain a better understanding or provide feedback on it. Full definition
Each chapter is followed by a short quiz for review purposes only. (myimprov.com)
The product was offered to me for reviewing purpose by «brand you love». (berriesinthesnow.com)
It depends on what the contract says, but pretty much all of mine have included a clause specifically excluding copies that were given out for publicity and review purposes from counting toward royalties. (jimchines.com)