I read the book a long time ago and I can't remember if there's any explanation to this, but in the 2017 adaptation of It, the Losers all just decide it's a great idea to walk right into the scary, abandoned house where they are pretty sure It lives. Did they have any sort of plan about what to do once they find Pennywise in there? Or was it just a plot device to get all of the main characters in danger?
It's pretty clearly explained by Bill that they all need to go to the Well House to both confront their fears and because It wanted to separate them, which makes them more vulnerable and less able to deal with It.
This is proven when they all decide to go their separate ways after their first group confrontation with It in the abandoned house on Neibolt Street, which almost didn't succeed because they all didn't go into the house. Not long after they go their own ways, Bev is captured by It because they had separated themselves. Therefore the only thing they can conclude is they need to work together if they have any hope of beating It.
Further allusions are made when It tells the kids they can go if they just leave him Bill, and Richie tells Bill it's all his fault these things have happened, "And now you're gonna make me have to kill this clown." They know if they let It keep Bill, then It will have successfully caused the kids to split up again, and there's no guarantee It will keep its (or is it Its?) word and leave them alone. Further, It will just pop back up in 27 years and cause problems all over again.
The first question is "did they have a plan?", and the answer is: no.
Bill was obsessively haunted by the loss of his little brother and once he had even a shred of an inkling where his brother might be, he felt he "had to" go there to save him. How? Never explained.
Any attempts to even start talking about a plan kept getting interrupted, like during the projector scene, or when the psychopathic bully shows up to literally murder them all.
At best, the plan was that if they're all together, they could hurt "it" somehow ... but even that is kind of a plothole because the hypochondriac kid was definitely freaking out when he got his arm broke, and was certainly not "part of the group" when the lady just jumped-in and stabbed "it" through the head.
The second question is "was it just a plot device [...]?", and the answer is: yes.
There was no plan, there was no explanation of where "it" came from or how to kill it. Any research that the history kid did was mostly ignored by everyone and they vaguely knew that it would disappear (probably) for 27 years, so now's (maybe) their chance.
Chance to do what? Who knows. They winged it. But apparently it all worked-out ... except for the dead cop and the literal dozens of dead children that they just left in the sewer and presumably told nobody about -- can't waste time on that, not when there's a pre-teen, blood-soaked, makeout scene that needs to happen.
As far as the plotting, pacing, and narrative of the movie: it was a wholly contrived and underwritten narrative that awkwardly forced all the kids to directly go confront a literal shapeshifting nightmare demon in the sewers under a well under an abandoned house, with no attempt to tell anyone or plan for anything.
At best, the farm kid brought the compressed-air bolt gun, but then the "steaks" were raised by having them lose all but one can of compressed air. And then the only other planned main weapon was when the loudmouth kid gave Bill a bunch of fence-posts that he never actually even attempted to use -- despite numerous chances. Conveniently, there was a baseball bat for the loudmouth kid to perfectly time his Columbo speech to.
Yes, in the contrived circumstances of everything, they did "need" to go there, and to go there together, to confront it and fight it.
But the question(s) were if it was just a contrived, plot-driving lack-of-a plan, and in that light: yes, yes it definitely was.
An interesting read can be found here, for the movie that might have been: http://www.vulture.com/2017/09/how-the-new-it-movie-deals-with-the-child-orgy-scene.html