At the start of any froum, you need members and posts.
You either have to get them from your firiends and family or people you know on line.
Or you have to fake some.
If you have a new forum with only one member doing all the posts, it would be very hard to get going.
You may as well use fake users to post around if you're considering doing post exchanges with other forum owners. It's just the same, saves your energy, time and potentially money (if you're paying for it instead).
If you're going to do it, make sure those fake users post insightful posts rather than just creating standard discussion topics that won't generate much traffic, if you get what I mean. It's really all about the content and how SEO optimized it is, traffic will follow and in turn, users.
Instead of using fake members, why not use a paid posting service (made up of real people) or do post exchanges?
They're not loyal members, but they're real people. If you're exchanging posts, you're helping out a fellow forum owner. That means something, especially now that forums aren't as popular as they were 15 years ago.
Using fake members, though, feels too much like a lie. Get caught lying to your members, and that's it for your reputation!
It's not necessary in my opinion. The best way to establish a community is to be well respected in a couple of other communities, and then to use platforms you already have to help spread the word and gain members that believe in your vision. Sometimes being small actually works to your advantage. Your new members will feel more valued and will feel like they are a part of establishing something, and that tends to be enough of a motivator on its own to get new folks to help create content (even on a brand new forum with only a few accounts).
Resorting to fake accounts isn't really a good idea in my opinion. It won't really stand the test of time.