D&D 5E - What's the point of Giant Insect?

The main difference is Counjure Animals doesn't let the druid choose what kind of animal you get. You might want giant spiders but you might get giant sheep.

Another, probably less significant difference is the "Animals" conjured are actually fey, not beasts.

Giant Insect can give you a giant scorpion, which at CR3 is more powerful than anything Conjure Animals can give you, or with 10 giant centipedes is a greater number of critters.

It's also about the worst named spell, since centipedes, spiders and scorpions are not insects.

So far as I can see, "Mighty Summoner" applies to both spells, unless you have a very pedantic DM.