The booster can supply 12 amps or 3 x4 amps to separate sections (layout divided up into separately powered sections).
It could offer an extra 12 amps to the central station's 12amps if needed as in if multiple locos are being run together with their lights, sound units and smoke (and also rolling stock lights etc) on a big layout.
I have used, for five years ,a DC 12 amp Aristo Elite power supply that feeds to 3 separate individual layout sections. On the main line I run up to quad headed USAT diesels with sound units and maybe smoke and sometimes with lit rolling stock. That takes a fair whack of amps... BUT the other two sections, sometimes, are also running at the same time and want their share of the amps so 12 amps can soon get gobbled up.
If I was to have DCC across the three sections I would keep and use the booster but, I will have least one section continuing with DC and the second section mainly DC so 12 amps DCC is fine for the main line. So no need for the booster. Also my layout is certainly not the largest in the world so track length isn't really an issue. Which leads to the next point.....
On running jumper cables:
Although track (brass but not stainless steal) is a better conductor than even 2.5mm mains cable, that is supposing that there is no loss around the layout due to the resistance with fishplates, point mechanisms etc (can be overcome by soldering track joints or using brass clamps). Once the resistance points are taken into account, the mains cable wins (especially if it is 6mm!) and it is advisable to run a jumper cable or two (especially with stainless steel which is a poorer conductor when compared to brass) if the layout is really big in expanse or has many loops etc.