ON PLANNING SETS (bear with me)
When DJing took off in Britain in the late 80s and ealry 90s hip hop was the culture that brought it. Yes of course the club/house/rave scene kicked off and thats what made lots of people get into it, but hip hop cultures influence was a high benchmark. Lots of early rave DJs could or would want to be able to scratch. integrating hiphop tricks (playing doubles to flange + drop back a half beat, even cutting backwards and forward between two tracks rather than a quick blend, spinbacks, dropping a track in not from the start etc etc) all came from hiphop and the DMC championships were of interest to many non hiphop DJs.
Planning a set, or to use the comedy analogy having some bits you could string together, was central to the best hip hop DJs. If you want to make your mixing into an artform you cant trust to luck, you have to work out some mixes and prepare and practice them. This remains the case for dance music mixing.
That era was also the era of the tape and limited access to music. Tapes would get played over and over and over. And a great mix between two records would really stand out and take on value. Well thats true for those who care about the art of djing, many people dont have a clue what a DJ is doing, leading to "pointless knob twiddling" comments etc. (yes there are fakers out there pretending to do something)
Playing spontaneously and seeing what happens is fine of course and can throw up some great mixes if you are lucky. But personally I find the vast majority of Djs, famous and not very, pretty pedestrian <absolutely fine, no problem with that at all. But when someone has actually put the work craft and art into it it elevates it - if you appreciate DJing that is, and hopefully even to a baffled listener. And that is so much easier now with digital than it is with vinyl: creative possibilities are huge now...its why I love digital DJing so much more than vinyl.
So yeah, nothing wrong with preparing sets, in fact great, perhaps the equivalent of a well rehearsed band at the top of their game compared to jam session.