Evoking the entities seems to almost be considered silly, a waste of time, or dickwaving. Why, oh why would anyone want to evoke an entity when you can just contact it using a means of divination or even astral hearing?
This is probably due to the fact that as a chunk of western magick was broken of it and then watered down into an unrecognizable weak mess, (kinda like budweiser compared to real beer) a ton of juveniles, or people who never got beyond that stage, wanted to evoke things as a proof.
Originally, the reason you evoked something was to make sure you actually got what you wanted to be there.
Some of my readers will be familiar with Afro-traditions. You will be aware that in Haitian Voudu the congregation dances together with the priests and thus anyone can become possessed, BUT because anyone could be possessed people are worried that someone might be faking it. Thus the Lwa, when they come down, not only say secret things and give signs, they also do things the person coulnd't do on their own. Sit on machetes, rub peppers on their genitals etc.
Now, from the point of view of the still living tradition that works with these entities, why would you want to evoke them?
Well, lets start by the fact that they are frickin' Demons. They are high up (or down) the specific chain. From the point of view of the tradition, you don't know for sure that you got that specific demon unless that demon shows itself. before that it can be a shade, an elemental or a lesser demon or anything else for that matter.
Just to put things into perspective: To simply communicate with a demon using a pendulum is to the Tradition the same insult as if you would have started to call upon Orisha during your LBRPs and about as effective for the original purpose. Or, lets say visualising yourself astrally as a Lwa and assuming their godforms instead of a full on possession.
Best of all? It can and often is, you.
Also, if I put it mildly, the purpose of evoking a demon somewhere is not to ask to get laid or for some luck gambling. If you get the demon to be somewhere for a reason you better know what to do with it.
Hint: some demons in Lemegeton are hoping to become angels again after a set time. ever wondered why?
Now, when we evoke something (unto visible appearance) one of the requirements is that others should be able to see it and the entity needs to be physically present for example to lift things. The thing is, even this can be explained away using mass-hysteria telepathy telekinesis...
Before you actually know WHY you ought to evoke demons, it might be useful being able to do it. The skill to cause mass-hysteria or the level of divine influx needed, or whatever you want to explain it as, is an indication to whether or not you can do what you need to do when Belsebub comes knocking on your door because some of his best generals suddenly grew wings again.