Image descriptions help visually impaired users engage with your content
That's a good thing to do on its own merits and is well covered in Molot's answer. The effort it takes won't cost you much and could help someone else.
If that's not motivation enough, adding them could boost your metrics
Visually impaired people aren't the only 'people' that will ...
When screen reader reads the page to visually impaired person, she will hear something like:
Image. Described as "enter image description here".
Details may vary, some readers can (could) be configured to also tell the size and is it color or bw images, but that's merely a bonus and not always in use as it wastes time (hearing is slower than ...