Varnish is a tough topic. Some do, some dont. But if you want your work to last, and colors to not fade, and dirt not to be part of your legacy, you better varnish.
A good varnish protects the surface from dirt and grime. It also protects the paint pigments from the destructive power of ultraviolet light. Both these forces are entropic, and do their job over time.
Varnish also brings dull paint out and brings the color to life. It unifies various surfaces, and clarifies.
Another thing varnish does is protect the surface from various mishaps, like scratches.
There are many, I personally use two, Golden's and Utrecht spray. Both are UV blockers, both are gloss. Gloss is preferred because the stuff used to make things non-gloss tend to fog the surface, and dull the paint. Gloss is also easy to clean. Both are also removable and repairable. All varnishes age and yellow, and being able to remove them and redo them over time is a big deal. It is also handy when a repair is needed. Do this long enough and you will need both. The varnishes I use can be removed and the paint underneath undisturbed.
The Golden is fairly thick, and I dilute it with clean non-oderless mineral spirits to a workable finish. Varnish requires that a painting surface be completely dry or cured before application. Otherwise, you may simply preserve the undry state underneath the varnish indefinitely. Varnish needs to be applied in a clean space as well, no chance of settling dust or particulate falling from an old ceiling as application is best done on a flat surface face up. Until a varnish 'sets' it may sag if the painting surface is vertical. Until it sets, it needs to be protected.
My observation with paintings is they go through a couple three stages of finishing. There is that point when they tell you they are finished, and that is a satisfying moment. They they get cleaned and varnished, and become a new thing. Then they get lit and viewed, which may include frame or not. That is when they come to life, if they are going to.
A quick note about framing. I think of it as a tool, the primary purpose of which is to cause the viewer to consider what is inside the frame more seriously. It separates the conversation from the surrounding noise, and causes you to see it separately. It intensifies concentration. I like them simple, unadorned, and neutral.