Many times the computer shipped with XP Pro, in later years, it was the most popular of the two main versions, as many downgraded from Vista Business to XP Pro.
In my case, it was my former employment. Ran Windows 2000 Pro for 6 years, so when I took the XP plunge, I went Pro, the same with Windows 7 & 8 Pro was offered for $15 & $40 when I bought it. Pro these days is considered to be an enthusiast OS & MS pushed it as such, not necessarily a work one. There is a certain level of prestige when having a Pro over Home versions (kind of like driving a Lexus over a Toyota), personally on Windows, I'd not want a "Home" OS. The one PC that I bought with Windows 7 Home Premium, I grabbed a Windows Anytime Upgrade key to make it 7 Pro, at the time, to have XP Mode.
The main thing is this.
It doesn't matter why they were running XP, they now want a operating system which will be supported.
The OP mentioned having XP Pro, but didn't state anything about requiring a Pro version of a new OS.
There are computers out now with 8.1, being that's what users will need anyway & that any bugs that upgrade installs has shouldn't be an issue, it would be best to get one with 8.1 preinstalled. Though there hasn't been any official word, it's likely that those with 8.1 will get preferred pricing over Windows 8, 7 or Vista when 9 is released. If it's going to be released as an upgrade, 8.1 may need to be there first.