if i am sending cryptocurrency from one BTC blockchain wallet website to an another BTC blockchain wallet website what is % of fee ?
You do not need to send and receive your own bitcoins. Instead, you can export and import private-keys so that you will have your own bitcoins in single wallet unless you want to have all your bitcoins in single address.
Altough, technically, you are correct, but i personally feel there are several reasons to have more than 1 wallet.
For example: spreading the risk, having a big chunk of your coins in cold storage, better privacy protection, the fact that it might not be a good idear to have your unencrypted private key stored somewhere,...
Personally, i have an electrum wallet on my desktop for day-to-day use, bitcoin core on my server, a ledger HW.1 for longer term storage and a paper wallet for cold storage