Under what circumstances do you personally use / need it?
first, you want the heat from the bed to heat the bottom of the object so it cools more evenly, in a boxed printer it's a different story but in open one that's super important.. if you print long pieces or thick plates (for e.g. cover screen for device you just made thats 5x8cm plate 5mm thick) and you don't want them to warp... you have to have bed at proper temp ... without preheating till the nozzle gets up to temp bed goes to 40-60C depending on previous temp, temp of the room etc... that's not enough for a big print..
what's more important after some 9 years of 3d printing (and hundreds and hundreds of kilos of plastic used) one start to look at rafts with resignation and disgust ... and if you want to print raftless (talking about ABS only, PLA is different beast and I rarely use it except for support on dual nozzle machines) that perfboard is a no-go so you have to use "smooth" boards ... many options exist... and they require different approach
- a smooth plate (alu, glass, whatever) covered with kapton tape
. this requires some 70-100C for proper adhesion but kapton gets messed up over time, is not cheap where I live so I'm not big fan of it .. it does give you great finish but.. also 70-100C is not something you can achieve without preheat!!
- a smooth plate (alu, glass, something with kapton on..) sprayed with hair spray
(you have to use cheap hair spray without parfume), this I personally don't like, with PLA works great with ABS and big parts you get warping issues if the bed is not hot enough, and even if bed is hot enough (and never is with UP even with preheating working) you get warping on some specifically difficult parts so .. easy to work with, I don't use it (if you go with PLA it is attm the best option - borosilicate glass sprayed with hair spray) .. 70-100C necessary for good adhesion for ABS, less for PLA
- same crap as hair spray only you spread paper glue (one in stick) on your smooth surface
... works great only there's HUGE chance of nozzle clogging so I don't recommend it with UP! (I destroyed one whole nozzle+heater block, 100$ in damages, because of this) ... 60-80C necesary to print
- ABS slury
- this is what I use now, make a slury (take acetone, pure one not the crap your wife uses that's full of oil and other crap, dissolve scraps of ABS in it, leave overnight to fully dissolve, you want consistency of milk, if you added too much abs add more acetone, if it's too thin, add more ABS wait another 24h .. don't heat up acetone, leave at room temp). use a smooth surface, I use 1mm thick alu board that I put instead of perf board, use a brush and coat it with the slury in one direction, leave it to dry, now go with brush but in 90 degrees strokes to what first coat was, leave to dry.. you have a print surface, print on this, 50C is enough for good adhesion (still you want hotter during print on open printer to relieve stress but I'm printing pretty hard to print parts on this without problem), 100C ideal .. when you remove the part after print you will remove the piece of slury with it (its thinner then paper, easily cleaned from part) use the brush and recoat the bed where the slury was removed ... The only problem with this tehnique is that your slury is usually some "undefined color" (usually starts white but as you use it you add more abs and more acetone in it, you add colored abs so it gets to some undefined "pidgin" color) so the bottom of your part gets some of that color on it. Ideally you could when you put a new spool, first make a slury out of that spool and you have the same color slury as your abs, and every time you change printing color you clean your print surfdace and apply properly colored slury but ... my life is not long enough for that so I use same slury always :D
- special print surfaces
for e.g. I, on my other printers, use printbite, it is crazy good print surface, CRAAAAAZY good .. but it requires 100C for good adhesion (100C to be the printbite, that's usually 120C of the bed it's on). up was properly heating so I could not use printbite with it (great thing with printbite, you heat it to 100C ABS glues to it like crazy, you cool it to under 50C abs detaches without any force) .. now with my new heater it does work but ... njah, never mind ..
hope this helps understand the preheat issue... so to summarize
- attm bed comes up to 40-60C when the print starts
- some print surfaces (most of them) don't find that temperature hot enough for proper adhesion object to print surface