The role of the saucer separation ability (or more exactly the reattachment ability, because supposedly all saucers can separate once, as suggested by "The Apple" and shown by ST Beyond) is never made explicit in Trek, is all. And unlike with the case of warp drive aboard (we sorta see it there a couple of times, we never get evidence to the contrary), information on the separation-reattachment itself is contradictory.
In "Farpoint", separation is used for taking part of the crew out of the harm's way, and we get two characters arguing that this shouldn't be done. Data tells that separation at any warp speed is inadvisable (debunking the idea of a "handoff" warp field), and Worf balks at being ordered to escape (confirming that flying the saucer is considered escaping by default, because Worf receives no orders beyond being told to command the saucer).
In "Heart of Glory", separation is dismissed as a pre-battle maneuver, but Riker does suggest it. For a rare once, it would be helpful, as the enemy is not yet in sight, and the supposed inferior ability of the saucer to fight thus would not jeopardize the escapees. Yet if it is intended Starfleet doctrine, why does Picard dismiss it?
In "Arsenal of Freedom", something ate a starship and yet the E-D again fails to preemptively leave the saucer behind. When separation and partial escape is commanded, Logan balks, feeling that total escape without separation would be the correct action. LaForge feels separation counteracts "risk".
In "Best of Both Worlds", fiddling with the saucer is a tactical ploy, without an escape aspect. Then again, escaping would be futile, both tactically (the Cube could catch the saucer) and strategically (having everybody die would be a small price to pay for victory, and saving a saucerful while the rest of mankind perishes would not help much).
In ST:GEN, the saucer is separated because the stardrive section is going to explode. The same is attempted at extreme warp in "Hollow Pursuits" but considered too risky because the ship is in poorer condition than in "Farpoint".
In ST:B, the saucer is separated because the rest of the ship is falling apart. There is a tech complication driving the separation: it will overcome a software issue where the saucer impulse engines won't work properly until severed from the warp reactor, and the heroes go for physical severing so that the ship (now just the saucer) will regain impulse and escape into the nebula. Further escaping at warp is not discussed, and it's unknown whether such a capacity would be built into the parallel-timeline ship's saucer. (Then again, we don't know the distance between this planet Altamid and safety - might be a short impulse hop rather than a warp journey for all we know.)
There is no other reasonable explanation for it.
Those can easily be made up. The saucer could be a base to be deployed on planets of interest, capable of soft landing and extended stay. The saucer could be an exchangeable and interchangeable module, to be replaced by something more combatworthy in times of war, say. The whole concept could be modular, with various parts combining in various ways, there being multiple choices for the lower part, too. Lifeboating would be a secondary option in all the above cases, and would thus rightfully cause onscreen debate (fine with "options") rather than automatic action (mandatory in "emergencies") when suggested.
The main point is, the separation function is dismissed or ignored more often than not, so we get very little data on what it was created for. Barring explicit references, we can come up with better excuses for why separation basically never happens than if we did have explicit references to an escape doctrine!
Timo Saloniemi