pacmaninbw wrote a decent answer already, but let's clarify this further by an overly long answer.
People come here to get their code reviewed. Enthusiasts, starting professionals and veterans, all on the same site. Advice useful to somebody without background in software may not be useful to veterans. Veterans are likely helped by different answers than enthusiasts. Regardless of how you would group people together, there'll be those with more and those with less experience.
So it helps both questioner, answerer and people coming late to the party to indicate a level of experience. Keep in mind, we're talking experience here, not code quality. I've seen questions by beginners that had both a higher code & question quality then from some who were at their 7th Code Review question in that language.
Now, there's a world of difference between a veteran and a beginner. It's not feasible to create and maintain tags for all levels of experience. What we can do, is help people who feel they're just starting to get their feet wet in a situation a bit different than we'd people who've been doing this for a while. I've used the tag myself a couple of times, whenever I wasn't sure about my approach in a language I wasn't familiar with.
Beginners are often helped by reviews that stick a bit closer to the basics and explain a bit more about the how and why. Or, to quote the beginner tag wiki:
Some adjustments a reviewer may make when reviewing a beginner's code are:
- Common practices and code styles that "everyone knows" may be absent, and that's OK. The reviewer will probably make note of these common practices without being "harsh".
- Assuming that language constructs used in the code are there because those may be the only alternative the asker may know, and not because they are being used for any special reason, may lead to suggestions to use different language features or libraries.
- Making suggestions that are easy to understand, rather than being the most efficient or optimal.
- Pointing out common patterns (not necessarily related to the specific language) that "everyone knows", but may not be obvious to a beginner.
- Explaining potentially new concepts in more detail than needed for more experienced programmers.
A lot of those points are specific to Code Review. After all, on Stack Overflow, you simply want an answer to your question. Perhaps with a bit of explanation, but that's it. On Code Review, you want more. Code Review deals with questions and answers that would be (among other things) too subjective and too broad for Stack Overflow. On the other hand, while most of the SE network accepts "how do I fix problem X" questions, Code Review only does so in a limited capacity (performance, optimisation and readability problems for example, but the main problem you'll always have to have fixed yourself first).
So why do we do things different than SO? Because we are different.
To help people get the right review anyway (no guarantees, but it does tend to help), we have the beginner tag.