Every time a new article about Grant Gustin from The Flash going over to guest star on Supergirl runs, the commenters inevitably come out of the wood work to say something like, "Oh, CBS is trying to save Supergirl" or "Does Supergirl need the ratings boost so badly?" or something to that affect.
And it makes no sense.
Here are the ratings for these two shows:
The Flash averages a 1.4 demographic rating. This season, (this week, in fact) its high was a 1.6 with 3.94 million total viewers. It's the top-rated show on The CW (for the record, DC's Legends of Tomorrow is #2, and Arrow is #3).
Supergirl meanwhile averages a 1.83 demo. The season high was the series premiere, an unrealistic number at 3.1 and almost 13 million viewers. Outside of that, it did also have a 2.2 and 9 million viewers. It's the #5 show on CBS, but The Big Bang Theory and NCIS are such massive stalwart hits, it's hard to even consider them. Regardless, the top 5 puts it above two NCIS's, and many others - 20 scripted shows are on CBS right now.
What does all that mean to people who don't watch the ratings for a living? It means neither of these shows are in any danger of cancellation, and neither of these shows needs a ratings boost.
So why are they crossing over? Use the old "Occam's razor" solution here: the simplest answer is usually the right one. These shows are crossing over because it's going to be fun. When you look at the initial announcement, it's very clearly right in there:
"We are so incredibly excited to announce something that we have dreamed of happening since we starting making Supergirl – The Flash and Supergirl are teaming up!" said The Flash and Supergirl Executive Producers Greg Berlanti and Andrew Kreisberg.
This was something that, since fans saw that Variety cover last May, they'd been asking about. During every interview and conference call with press, the EPs got asked that same question again. And with The Flash introducing alternate realities this season, the opportunity presented itself in a way that CBS, The CW, WBTV, and DC Entertainment could all agree upon.
When fans saw things like the 90s Flash, Melissa Benoist as Supergirl, and the Legion ring in the tunnel between time and space, they didn't try to figure out what the ratings would be (spoiler alert: they were awesome, too), they were just excited about the nods. And if you've ever talked to, or read or heard interviews with, Berlanti and Kreisberg, then you know that they're excited, too.
These shows don't need your worry, they don't need your over-analysis, they just need you to have fun watching them. If one of them isn't your thing, hey, that's okay, maybe the crossover episode will be fun for you anyway. Maybe it'll make you give that other one a second chance. Maybe it'll give both a nice ratings bump. But that's not the point, or even the goal of the creators, it's about fun.
Why not have fun with it, too?