"Surely anyone only ever buys something because they like or need it and can afford it."
^ Do you really believe this in today's consumerist society? Personally I only buy things I can afford but I'm lucky to be financially stable. Some things I buy because I need them, many things I buy just because I want them.
The problem with IG where so many things are gifted is that it creates a false message as to an item's worth. Say a blogger does a "February favourites" video showcasing items they haven't paid for, including a £200 cashmere jumper. I can accept they might like said jumper, but would they have spent their own hard earned money on that particular jumper if it hadn't been a gift? More likely they'd have bought a cheaper version, or made done with one they already had, or, if they really though it was worth it, saved up or cut back elsewhere.
By not disclosing that they've received it for free they're giving viewers the impression that the item is worth spending the budget they have on it, whereas the bloggers' own budgets are not limited by the same constraints as they're being sent all sorts for free.
I would hope most grown adults are sensible enough to see through IG but I do worry about the influence it has on children. They receive the message that it is desirable to have lots of things whereas the bloggers who are spreading such messages don't have to pay for any of it and, in some cases, are then selling gifted items on to generate income.
So in a nutshell it does indeed matter to many whether things are gifted or not.