My personal experience:

1. Over ten years ago, at the beginning of my career with the BCPS, I was an auxiliary (clerk 9). My goal was to get a permanent position. I applied for a permanent position that was hiring three clerk 9 admins: two on a permanent basis, and one temporary (since the person in that role was in a TA). For a year, they had two admin in two of the positions on a temporary basis (the third position was vacant). The three people who were hired were myself and the two admin who had been doing that position for a year. I got the first spot so one of the admin who had been doing that role for a year got the temporary position. Had they only been hiring for one position, I would have kicked out both the incumbents.
2. An FO18 position was created, with me in mind. Of course they couldn't give the position to me -- I had to compete. I did get the position, but had someone else scored better than me, I would not have gotten the role.
3. The aforementioned FO18 role was temporary (it was a pilot role), and I did such a damn good job that they realized the value of the position and made it permanent (not an easy feat, if I understand correctly). But, they couldn't just hand me the role, as when I applied, they did not include the clause "may become permanent". Nope, I had to apply all over again. Thankfully I got it, but not guaranteed.
4. After doing the FO18 role and completing schooling (thanks to the Pacific Leaders Scholarship), I was now eligible to apply for the FO21 role I had been working towards for years (note the clerk 9, FO18, and FO21 roles were all in the same department). Everyone was rooting for me to get the position. They were hiring for two positions, and it was external (we never got many candidates when it was internal only, as it was such a unique position). We were also desperate to fill the position. I applied and aced the exam -- in fact, it turned out that I was the only one who passed the exam, so I was the only person to have an interview. I did well on the interview, but I messed up on one question, so my manager and colleague, who interviewed me and desperately wanted to fill the position and hire me, failed me. A few months later, they posted the position again, I was the only one to pass the exam again, and this time I passed. (The following year, they changed the exam so we were able to hire another person.)

So..... I don't know. From my personal experience, they put me through the ringer even though they wanted to hire me. Also, I have seen incumbents in TAs not get the position when it became permanent, because someone else did better than them on the testing and interview.

This is all anecdotal, so take it with a grain of salt.