In the past couple of weeks ants have become very active.
Down on the ground some Fromica workers scavenge for expired honey bees from our hives.
Fitting them in the entrance takes some effort but it's well worth it for the creamy center. Expired honey bees are bees who's wing muscles have given out. Documentaries say they "turn their backs on the hive" but really the honey bee is still trying to do it's thing. Hell after they've stung you they still try to sting you even though their abdomen is split in half and they're dragging their internal organs behind them.
Up high the ants are foraging for nectar and delicious caterpillars in the trees. As plants grow they secrete small amounts of sap and extra floral nectar. Ants help clean this up, though I'm sure something else would have if they didn't do it.
Occasionally they'll find aphids, as not photoed above, and hang around the tree longer. Most species aren't a huge predatory force for most bugs but will harass other insects enough to drive them away. Caterpillars are usually easy targets for ants but I'll point out that many caterpillars have evolved defenses.
Flowering plants that don't have defenses against ants end up losing their nectar to them. It's not that major of a problem usually. Nectar and sap on the new growth is a big enough food source as it is.