There’s a few spells which secretly unlock a class feature; that is, if you take this spell you are working the way that the system intends, and if you do not, you are not.
They’re often easy to spot: only one class has access to them, they’re “better” than other spells at that level, they take a bonus action to cast (so that they can be combined with the “main action”), or have an all-day duration as a balancing factor.
The warlock is particularly guilty of this: eldritch blast is the best cantrip because it uniquely scales to multiple targets, does at-will damage of the single best type in the game, has a great range, does excellent damage, and is built into several invocations. And they’re the only ones who get it. Hex, too; it starts scaling past one hour and bam, you’re chaining short rests and keeping your concentration up, and note how well its “1d6 extra damage each time you hit” combines with “eldritch blast hits multiple times”. It’s a damage buff. I’m not saying you’d always cast hex; it has the opportunity cost of concentration after all. But I am saying you’d always choose to know hex. The warlock has many other unique-access spells; those don’t bother me nearly as much, because they do something that doesn’t feel like a mechanical contrivance.
The ranger’s equivalent to hex is hunter’s mark and, like hex, it feels like a class feature. They’ve got some other flavorful spells here and there, but only hunter’s mark has that same “would you like to do more damage or not?” aspect to it; I really like hail of thorns but it doesn’t feel like a secret class feature to me, because it’s not clearly intended to last all day (concentration willing) and doesn’t provide that little extra buff of damage. Especially for an archer ranger, the parallels are downright eerie.
The paladin’s find steed seems similar in a way. I’m really quite shocked we haven’t solved our ranger conundra with a find companion, given how similar the use cases are; in my campaign, the ranger has access to find steed and can absolutely use it to pull bears and mongooses (mongeese?) too. If your paladin wants a horse, you cast the spell and have a permanent magical friend-horse from a list that’s, frankly, not too shabby. Give it some spell level scaling and we’re in business. What’s interesting about this case is that the paladin is a prepared spell caster and, like the cleric, it pulls from the full list; there’s no opportunity cost to this feature unless you want to cast it in a weird situation.
Contrast this with the wizard’s find familiar and mage armor. Actually, given how easily spells come to wizards and the fact that find familiar is ritual-castable, that’s obviously a class feature with no downside to speak of. Mage armor‘s 8 hour duration makes it feels similar after a certain point, though a 1st level caster is so constrained and a caster’s place should be so far from the front that it’s not a gimmee.
The out of combat class features — find steed and familiar — rankle more than the in-combat ones. The in-combat ones feel like other character types should be able to gain access to them; they usually cost a resource (even if, like eldritch blast, it’s a tax), and they unlock a capability. By the time they start getting 24 hour durations, the need to concentrate becomes quite frustrating (last up to a day — and down to a single turn, depending on dice!). But all in all, I can see the need to make it a spell. But find a friend, what is WRONG with you?
Just a rant, nothing constructive.