Sure sometimes activists (you know, people) get excited about a campaign and pour money into a probably unwinnable campaign. And, hey, sometimes the people "in charge" do much the same thing. The former at least has the advantage of getting people involved and excited about something, while they latter often involves throwing money at someone based on the the fact that they're proved their worth by being rich enough to throw money at themselves.
That isn't to say I think primarying Manchin is an especially good idea, but if the right candidate appears, why not? The DSCC and DCCC throw big money at losing campaigns every cycle, though not primaries. Winning is obviously the most important goal, but it isn't the only purpose for a campaign. And there is no lump of campaign resources (money) to be allocated. If people get excited about something they contribute, and it's not as if that money would go elsewhere.
Anyway, I guess I always see too much fretting about these things. If a candidate can get attention and raise money and get people involved, that's good, and not a waste of fixed resources.