I'm not sure how loud the 'in-situ' desk is, but the XR18 is only feeding a line-level signal into whatever it is.
I've only ever plugged into another desk 3 times. The first time it wasn't me that set the PA up (the rest of the band did) and they plugged it into two mic-pre equipped channels. Feedback, weird sounds that I assumed were impedance mismatches and awful phantom frequencies were the main problems, but I quickly identified that the desk had a Power Amp In so I plugged our desk in. Interestingly it was our old desk that had XLR and Jack outputs so it was easy, but the XR18 doesn't have Jack outputs, only XLRs.
BigRedX's comments were congruent with my experiences and the DTI seems like a budget solution that has effectively given the XR18 jack outputs. Out of the solutions of a pair of Class D amps to build a bespoke power amp, normal cables that might result in the issues that I've described above or perhaps custom cables, the DTI seems sensible.
I'm not resolving a problem that I've encountered with this set up, but given that I'm the guy who's shouldered the responsibility of getting us sounding good, having this solution (also phono outs in case we need to connect to a DJ mixer) seems like I'm pre-empting problems that I've encountered in the past.