With traditional towed guns, the 'purpose' of staying put doesn't exist, its more of a necessity. In theory, you'd still want to pack up and tow the gun away to a new firing position after shooting. In practice, the process of taking down the gun, towing it, and setting up for fire at a new site takes way too long, and if counter battery is paying attention, you won't finish that process in time. Traditional artillery tries to spread out, dig in, and generally is difficult to use offensively as a result, being fairly helpless during all that. Used defensively, being spread out and dug in means counter battery needs to land a direct hit inside the gun pit to reliably kill the weapon. Mechanical accuracy limits (Drones don't care about this as much now) and the natural inaccuracy of measuring artillery trajectories make that difficult, it'll take time and luck to kill the weapon.
Self propelled guns are basically superior in every combat relevant way except one - For the same cost/industrial effort, you get more traditional guns, and numbers matter a lot in artillery war. Creating self propelled guns ain't cheap or easy. You need to create a lightly armored vehicle capable of moving a very large, very heavy gun, at combat speeds. The vehicle also needs to survive firing a very fucking large gun, and be capable of doing so on various terrain under combat conditions without tipping over or burying itself in substandard soil. Operating and maintaining them is more difficult too, with the additional mechanical complexity from the ammo handling and aiming systems. That one only really matters if the gun survives, so isn't really a combat negative.
