I'd add to the responses above that, at least in part the logic of getting the 99s was that they wanted more 66s but couldn't get them (new build isn't possible, and there is only a limited supply of existing 66s available).
They then went looking for a locomotive that could be used on many of the flows they use 66s on, and the 99s is what they came up with.
Clearly, a 99 isn't the same as a 66, but it is more powerful on electric, and decently powerful on diesel, so as long as the routes they are used on have OHLE at the locations where the most power is required, you could use a 99 instead of a 66.