people are using so many bitcoins wallets just because they dont want to have all of their coins just in one wallet because not every wallet is very trusted by people so that is why they use more wallets to be sure that not everything can be stolen.
the most of them making a lot of bitcoin address not to looking for the safest wallet. when you think they make some wallet in many provider wallet bitcoin service to search which most secure while they was found the best secure bitcoin wallet they will stop use the other and focus use the best choosen. in my point of view why the people make few wallet is they want to manage their bitcoin in segmen. the examples one wallet especially for trading, and one other to receive payment from signature campaign.