Why? Don't remember.
Seriously! I haven't used one in years so I forgot. I remember one used to crash my browser, another slowed it down and leaked memory, a third one turned out to get sponsors in exchange for spying after the users, I just stopped trusting them all together and then came out the EFF's privacy badger and solved both ads and privacy. Yes, it has no slick UI for adding ad sources from a page, but I get to select which URLs are blocked, or just blocked for cookies, etc.
So far it's been stable and useful, I had a very short list of sites that would break if I had not excluded them, Most notably the SSO tricks for cross-domain cookies on sites like Flickr and Tumbler, but I have not seen too many people still using them actively these days, so chances are you would ont notice.
If you really want an ad-blocker, here is a random list to look into that I found for you on DuckDuckGo (As a privacy chauvinist, I haven't used Google search in years either)
Ad busting on YouTube and all around YT improvements provided by Enhancer for YouTube. It adds lots of useful controls and features to YT that I find hard to live without now. Speed control with your mouse wheel, lock minimum quality, dark or colored themes, default larger player, you name it.
Even more? take a look at SponsorBlock for YouTube, it will skip in-video annoying bits for you like sponsorships, self-promotions and non-music openers to music videos, etc. How does it do that? Well, it's the wiki wiki way, users like you mark segments and send to a central service, others can later vote them down or mark in new ones. Once you install the plugin, be sure to go into the settings and select autoskip only on ads and stuff you don't want, as the defaults skip a bit too much to my taste. e.g. I love Stephan Colbert's "Meanwhile" segments, but if I leave the defaults on, I miss the banter with the band at the beginning and the daily changing segment intro bit.
If you ever need to download videos, there is either Download Helper as a plugin, but sometimes it breaks (like when running Firefox as a snap on Ubuntu), so check out other methods on http://deturl.com/ or https://10downloader.com as fallback.
Finally the very useful Global Speed that works literally on every site where there is media to play, not just in YT. I turn it off in music streaming service tabs, but it will work on any other site even if the media player there has no speed controls. Simply magic :) BTW, if you are one of the weirdos that don't use Firefox, here it is for Chrome and for Edge.
Tangentially related, you might wanna try peertube companion