Algae is usually a plant and comes in different types.
Often, if conditions in your tank are good for algae, then you run the risk of encouraging some of the less desirable types.
(I say 'usually' a plant, because there is so-called blue-green algae, which is actually bacteria and blooms of this can be toxic).
Then there's brown algae, which is essentially diatoms. These are super-fine and tend to either form coats on your plants and fixtures, or can produce wispy cloud-like masses, wafting in the current.
Other algae is the easily recognised green hair algae, which shouldn't be an issue...and then black, or beard, algae, which often is.
All algae produces oxygen, in fact, diatoms produce something like 50% of the oxygen we breath.
Being plants, they can also participate and assist in the nitrogen cycle of our tanks.
...And algae makes great food for a variety of our fish and not just the so-called algae-eaters.
All algae can also become a nuisance in that it can foul the tank and your higher plants, your airstones and filters. It can also become unsightly, especially if you can't see the contents of your tank for the stuff. I've also found it to be a great visual indicator of the general health of my tank, although it is one of those things that will need regular maintenance and will appear, whether you want it to or not.
Going back to the black, black-beard, or beard algae, this is usually less useful than the green stuff and is the hardest to eradicate, especially since algae eaters won't usually touch it. It's as hard as nails and can quickly become a serious problem, smothering your higher plants. This stuff is actually a red algae and sticks to everything like muck to a blanket. As with all algae, too much can cause an imbalance in your ecosystem, with detriment to the fish.