Why do some journals want figures at the end?

There is a long tradition of submitting text and figures separated. this stems back to when manuscripts were typewritten and figures were drawn by ink. Many journals have kept this format and now the reason to keep figures separate is to facilitate the typesetting/layout. In addition, many electronic submission systems assemble manuscripts by merging text files with graphics files. The reason for having separate graphics files is that the typesetting processing inserts figures during the process. Not all journals take camera-ready manuscripts (in fact none in my field).

As a reviewer, and if you review from a printout, it may also be advantageous to have figures separately since it is easier to look at them in parallel with reading the text. If you review on screen, it will of course not be an advantage.

There is actually no need to format a manuscript this way as long as figures and text are delivered separately in the end if that is what the journal requires for their typesetting. Always follow any instructions provided by the journal!

You are correct that this has to do with pre-online publishing practices. In "the old days," most images had to be made "camera-ready," and were to be shrunk down to the required size. Consequently, the figures should be on a separate page at the end of the paper, so that different images didn't "stray" into the area that was to be photographed for another figure.

In spite of this, there has been no real need for the journals to change this practice, and thus it persists to the present day in a number of journals.