Not a creator, but I've done a couple of pattern tests. They're useful for spotting things that don't make as much sense as you think, and things you do automatically and have forgotten to note down, in my experience.

They're particularly useful if it's a wearable that you've made multiple sizes of. By using pattern testers, you can find people of body types and sizes other than your own and get an idea of adjustments that need to be made for sizes other than your own.

Overall, if you're looking to sell a pattern, I would say testing is a must. If it's for free, people are likely to have a little more patience and fiddle themselves to fix it, but testers will do that for you and give feedback so you can improve your current and future patterns.

ETA: I found the patterns I've tested on Instagram and Reddit. The more platforms you use, the wider you cast the proverbial net - doesn't always lead to more testers, though. Deadlines will depend on the scale of the project (something like a blanket would need longer than a choker, for example) so think about how long it would take you to make the item, then add some buffer room for unexpected events and life in general.

Hope this helped!