Usually, monster SLAs fall in three categories:
1. Defining ability. The monster wouldn't be the same without it. These abilities will often be the reason why you're using the monster in the first place. In other cases, these abilities are part and parcel of what the monster can do, or are just that iconic, anyway. Definitely use these abilities (or at least keep them in mind) if you use the monster, else it just won't feel 'right'!
Examples: Aboleth's various illusions; Treant's Animate Trees; Efreeti's Grant Wish; Night Hag's Etherealness; Duergar's Enlarge and Invisibility; Kraken's Control Weather and Control Winds; Annis Hag's Disguise Self etc.
2. Just-a-whole-bunch-of-stuff: many many SLAs, often usable more than 1/day, or even at will. These abilities give the monsters that possess them lots and lots of extra options, none of which can be said to be 'typical' of the monster in question. Taken together, though, they provide quite a bit of versatility and tactical flexibility. Some may buff the monster and its allies, some may provide an escape route, some are purely offensive and so on. This basically makes the monster feel like a secondary (or in some cases even primary) caster. You will very likely use some, but never all SLAs of this kind of monster.
Examples: Many Outsiders, especially Devils, Demons, and Angels, but also stuff like Inevitables, or the Barghest and Couatl; Green Hag; Half-Celestial/Half-Fiend creatures; most Genies; Lamia; many Fey; Ogre Mage; Nightshades etc.
3. Purely fluff abilities: some monsters have SLAs that are pure fluff and don't really do much to make them more powerful. In many cases, these SLAs are only usable a very limited number of times/day. Using these in combat is often a waste of an action - but on the other hand, they just might come in handy from time to time. Mostly, they're used to establish the creature's identity in non-critical situations, and for roleplaying or occasional utility.
Note that these SLAs might actually be the reason why your Conjurer or Druid summons these things. As I said, they're handy. Just not that good in a fight.
Examples: Stone Giant Elder's Stone Shape etc.; Aasimar's Light; Drow's Darkness etc.; Unicorn's Cures; Lantern and Hound Archon's Continual Flame etc.; Red Dragon's Locate Object; Formian Worker's Cure Serious and Make Whole; all creatures with a Detect XYZ ability.
Note that the separation of (Sp) abilities from (Ex) and (Su) abilities is completely arbitrary if you apply the categories I formed above.