I agree that, from an outside perspective, it seemed like an extremely weird and random thing to do. It may have made perfect sense for Bruce and his people, but I sure don't see it. Going through the trouble of traveling to Europe, organize a gig with Niedecken's band, performing "Hungry Heart" multiple times, then using only the vocal... a rather poor vocal at that... Maybe he was going through one of his depressions and wanted Jon Landau to book him a gig... any gig... and this was what they came up with. This is purely speculation on my part, but it's as good an explanation as any that I can think of.
I remember when this gig happened, there were tons of rumors of this being only the beginning of a surprise club tour of European cities. I was almost panicking. No one knew where he would show up next. Of course, there was no next.
I can't help thinking that this event was a sign of Bruce being rather lost in the years after the 92-93 tour and having absolutely no idea what to do. He'd seen his popularity plummet. Then he got an unexpected hit with "Streets of Philadelphia". He recorded a new relationship album that he or the record company obviously didn't believe in, because it was never released. He briefly reunited the ESB and, according to strong rumors at the time, considered a tour with them. He must also have been working on TGOTJ... several projects that pointed in completely different directions. Eventually, he chose going the solo acoustic way, which I think was probably the right thing to do.