It depends on how common you'd like to make the magical and divine interactions in your setting.
In this kind of setting, the divine powers of other Planes don't send entities or servants of their own accord - if they appear, it's because someone else summoned them there. The interesting part to this is that it means summoning is a big deal - you're pulling things into our plane that are exceedingly rare and very few people know enough about to have a bead on how to deal with them.
Obviously, no info makes combats interesting, but it also ends up opening up a lot of setting influence as well - imagine if someone in our actual world summoned the lowest of angels who could tell followers of a faith specific details about their afterlife awaiting them and the glory of their deity? Or to answer some ethical questions?
For arcane casters, imagine what kind of access to magical knowledge some of these beings have that they don't- they'd be able to advance their spell lore by generations... if they can get them to agree to share.
Old school D&D modules sort of go this route - although nearly every adventure had some kind of demon or elemental or something extra planar, it clearly didn't "hop over" on it's own accord and deities send in intervention rarely if at all.
The Prime Material Plane is an excellent crossing point, and maybe cosmologically important for it's own reasons, so all kinds of creatures are travelling through all the time. The important thing to remember is that all of these creatures need to have access by some means - something has to be able to Gate them in, and that might be a deity, wizards, more powerful angels/demons, themselves if they're strong enough, or they may have been able to cross over if there's open portals sitting around.
In this case, the Prime Plane might be an unregulated zone where everything crosses through and it's a violent free for all, it might be a battlefield of proxy wars, or it might be a place where you find normally opposed creatures working together or laying low. ("Wait, why is the a high angel and demon working together?" "They're lovers. It's quite romantic. Except the part where the forces of 2 deities both want them dead.")
As you go later into D&D, you see more and more high interaction settings and modules - where the extra planar creatures become more common - either sent by deities or having found their own ways over through portals.
My Personal Campaigns
I've run both types. At this point, I'm more of a fan of low interaction settings - I like the PCs to get a chance to feel like big players as they get to higher levels and if you have deities and outsiders constantly pouring in, it feels like the PCs are tiny grunts in a big war, even if they're pretty high level by all other accounts. It also becomes harder and harder to justify normal human survival if you've got all these uberpowered things just stomping about all the time.
This also almost always ties into how common you make magic in your campaigns as well - if every city has one or several high level casters, you have to assume planar travel and summons is relatively common. If, like in a lot of older modules, a lot of places have a single 1st through 3rd level caster, and anything higher is pretty rare, you don't have to ask how society isn't overwhelmed by magical options.
The questions you start ending up having to answer revolve around "So why hasn't a higher level caster already done X?" with "X" being some kind of massively overpowered world changing action. See this thread that details "Lunar Lich Wars" as an example.
The more gonzo you go, the more fantastic your world becomes, but it also makes it harder to justify how it works, which is a problem for some groups and not at all for others. Go with what works for you and your group.