A YouTube video I watched recently claimed that summoners might have been considered a bad class in second edition. However, it doesn't seem like that at a glance - there's opportunities for various creatures and beings to stand by your side, such as a plant monster kinda like Blossomon, a stegosaurus, dragons, and so on. Also they come with a nine level magic collection on top of the Eidolon - and the Eidolon can learn spells too with the correct feats. So you could make a druid like character with lots of magic but their main aspect being able to summon a giant sunflower, as a result of acquiring lots of Druid archetype feats for magic casting and feats related to the Eidolon casting magic themselves. Or you could make a stegosaurus man who summons a powerful stegosaurus that knows a lot about nature and fights for them. Sounds like a cool class to play, but I haven't tried the game much to see how any of this would work in gameplay.
They're how spellcasters get over the fact that they only have 2-4 spells per level and maybe 5 encounters per day. Each scroll is a consumable spell slot that you can use so long as you've got basic spellcasting ability, even if thats just cantrips and the trained proficiency in one tradition.
Unlike in PF1e and D&D, scrolls actually use your spellcasting DC, which makes them viable in combat against enemies you'll face in your adventures.
The lowest level ones are crazy cheap and can be applied to a wide variety of situations, so it makes sense to stock up on loads of them. When your budget for consumables is 500, why not buy a hundred lv 1 scrolls? fear, gust of wind, command, illusory object, etc. etc. can easily become as "at-will" as your cantrips.