If you just plan to use a desktop machine, there is NO reason for putting significant HDD resources outside the box. Use internals. BUT if you plan to heed all the good advice and create / use a good backup system, an external drive is a great pieve of hardware for that. It also can be useful as a means of carrying a LOT of data between locations if you need that for some reason.
Few connections between a computer and an external drive are as fast as the internal SATA system. So for simple operations, internal drives with SATA connections are faster and much simpler, and cheaper.