After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable.
The key point is "from the first one that exists and is readable". It won't read and execute all of them but only the first one.
The alternative was to have multiple scripts or stylesheet links on one page, which would trigger multiple HTTP requests. Multiple requests mean multiple connection handshakes for each link “hey, I want some data”, “okay, I have the data”, “alright I heard that you have the data, give it to me” (SYN, ACK, SYNACK). Even once the connection is created there is a feature of TCP called TCP slow start that will throttle the speed of the data being sent at the beginning of a request to a slower speed than the end of the request. All of this means transferring one large request is faster than transferring the same data split up into several smaller requests.