I use it for link research and competition analysis. I use it only when I'm trying to figure out an SEO issue with my sites (e.g. a penalty) or when I'm doing keyword research for a new site.
You need a subscription if you do lots of backlink profile analysis. If you don't, a free account will suffice. If you want to analyse the link profile of your site only, you can verify your site and gain access to the full tool. Otherwise, you will have to make do with the limited version.
Is it the number 1 tool I should use to get the backlinks that my competitors have?
Meh. It depends what you are using it for. I build pbn's and buy expired domain names. Therefore I need to analyze these domains to make sure they have a clean profile. I don't want to waste my name making pbn websites that have spammed backlinks. It's necessary for me to have a backlink checker.
For my purpose I recommend majestic seo. It is cheaper, and it measures how trustworthy your websites back links are. Ahrefs finds backlinks quicker, but they don't have trust flow and citaion flow metrics. Plus ahrefs is expensive.
I was thinking to buy it for copying my competitors backlinks, what you think?
If you are building a pbn and need to check the quality of expired domains, you need ahrefs and majestic to do a complete check. If you are not building a pbn, then I would not bother. You can get a few free searches per day using the tools in case you need them to check competition. Moz has good metrics but far fewer backlinks so I would not purchase unless you need a bunch of metrics.