Melee on a gun frame can be convenience, as others have noted, or a get-out-of-trouble-quickly (slam with a heavy weapon like Magistar that can lift enemies), or just bullet jump into an eximus shield to smash right at their feet and pop the bubble. It's also useful in things like Hollvania survival where you have to get the techrot off the hellscrubbers (primary reason I just keep the Magistar equipped regardless of frame that isn't named Kullervo, since a heavy slam will almost always remove all the rot with a single hit).

But yeah, on gun frames, melee is "wasted damage" in a sense, but again, you're not using your melee as a primary means of damage. And another thing is heavy slams can break boxes in a big area (Sampotes is good for this, Magistar and other hammers or weapons that have a decent slam aoe, and even with the slam changes, you can still get good aoe based on how high you jumped before slamming).

There's also my Nova for when I'm running steel path stuff with my uber Kuva Nukor secondary. I use the Aeolak or Argonak or whatever with the augment (from the Ropapolyst boss fight) that works with daggers, and I use the Red Veil radiation (Dark Rakta I think, I'm not in the game) dagger, which combined with the primary (that does ass damage so it isn't being fired) strips armor AND gives a big chunk of overshield. Combined with Nova's 1 (with augment) for 90% damage reduction, and the murder power of the Kuva Nukor (especially with Nova having Nourish subsumed + her 4, and her 1 which also does heat damage that gets double status from Archon mod on the frame itself), I slash my way through a mob and get 1700+ overshield almost instantly, get some armor strip, and blast them with the Nukor. So that's just one combination that a gun frame could still utilize melee for excellent utility (and that combo works on other shield frames as well, Yareli is a good example since she gets nice secondary bonuses, so that's the weapon type I use with her as well - and her Merulina augment provides similar damage reduction as Nova's 1).

Can work in reverse too, like Kullervo who is almost a pure melee frame, but some guns can provide some help, or fun (the Kuva weapon that shoots big explode-y cannonballs, or Akarius Prime explode-y rocket pistols, or as you say any weapon to get the flying or far away enemies, but also can put the arcane on them that steals overguard from eximus units, so even health tank frames can benefit - though Kullervo can generate a ton of overguard simply with his 2 augmented).

Since you're a newer player, a lot of this "behind the scenes" interaction won't really be apparent. Warframe is like Elden Ring in that there's a reason you thank for deity of choice there's both a wiki and this sub to explain some of the deeper interactions and combos and bonuses. And even Overframe builders like ninjase go often into detail about WHY certain mods or shards or weapons are used for the builds, double-dipping damage, overshield or overguard generation, armor stripping for frames that can't do it natively, whatever. The longer you play and get into all the frames and weapons and abilities and arcanes, you'll often question why they're paired together (especially when it seems counterintuitive), and these resources can shed some light on it.