Aerate isn't as great of an App as it could be.
Aerate is a Macintosh image compression program. It's design so that you simply drag and drop an image and instantly you get a quailty image.
There are 2 compression engines:
Opti - Faster Compression. (Not a whole lot of information on this.)
Zopfli - Higher compression into gzip and zlib formats but takes a bit longer than Opti. You get better results, but it will take more time to get there.
You can convert PNGs into JPGs, which can come in handy when wanting to convert a bunch of PNG and JPG images.
ShortPixel is Better
I tested Aerate against some other image compression programs and here are my notes:
Aerate wasn't able to compress all my images. Some of the images actually went up in size, but Aerate let me know that "no space saved this time." However, using the exact same image on ShortPixel, I was able to save 57%.
Screenshot to show ShortPixel was able to compress a particular image where Aerate said that it couldn't.
Aerate is free, the Pro version cost $14.99. Aerate Pro gives you access to "Fast" and "Extreme" compression modes. In addition, you get to use the Multi-core CPU to compress a lot of images. In addition, you can Bulk-compress multiple images.
I got Aerate as part of a BundleHunt promotion. I paid about $5 for the Pro application.
Honestly, I don't think it's worth the cost. ShortPixel worked so much better, I would recommend using their image compression.