"Projections" is an episode that might have seemed a little random, had later developments not kept it relevant, even if viewers had to wait four seasons to find out how. This is the first appearance of Reginald Barclay in Voyager.
He serves as a surrogate for the audience who otherwise gets to delve into the psychology and background of The Doctor, as we learn officially for the first time the details of his creation and creator, Lewis Zimmerman, as the surreal story plays with the nature of reality and tries to convince our Emergency Medical Hologram that he's a real boy. It's fun to watch the first time around, but takes on added significance the more you look into it, an episode that might have seemed pretty randomly episodic at the time, but even then had more significance than it seemed; I can only imagine the giddiness in the writers room when they realized later on that it made an incredible amount of sense to bring back Barclay, give him a second life, and actually make him more important than the memorable supporting character he already was.
It's also worth reflecting simply on the fact that "Projections" might originally have been considered a rare opportunity to see a familiar face on a series that seemed to have made that fairly impossible originally. The appearance of a character from one series to another is always interesting, especially when that character isn't even a series regular. Barclay was always bending the rules, though, wasn't he?
franchise * series * essential * character
Memory Alpha summary.