I'd be cautious to say that a private adjuster is not necessary. Yes, one could hire a contractor (for a similiar fee) where the fee is deducted from the cost of repair, to do an estimate. As for how a private adjuster charges, that can vary from place to place. In some state, for example, the fee is limited to a certain percentage of the estimate. For others, it's a flat fee.
What I have seen, mostly in areas where there is substantial damage as a result of hurricanes, tornados, and/or earthquakes (among others), is that the contractor is not 'schooled' in how to do an assessment. It's a similiar analogy that I recommend to people that they ULTIMATELY use a Home Inspector to check out a home IN ADDITION to their best friend's assessment, where their best friend is, say, a general contractor. (Just using this as an analogy and an example.) Neither here nor there, situation depending, do one or the other or both.