If you have plated-through holes, tracks on both sides of the board (and any internal layers) are connected without any further action.
Holes exclusively for this purpose are called "vias" and may be smaller than regular holes for component leads.
This makes manufacturing any board too complex to be single-sided, easier and usually much cheaper than otherwise possible, since no extra effort such as inserting jumper wires or soldering on both sides is required.
It also makes design and layout of double-sided boards much easier since you no longer have to strain to minimise the number of tracks on the "other" layer, or minimise the number of jumpers, or ensure crossings between layers aren't underneath components.
And that allows you to increase the board density and use smaller boards, cheaper enclosures, etc...
It also allows the PCB manufacturer to perform "bare board testing" of every one of these interconnections before any components are added - thus eliminating many defects. (Some PCB makers perform bare-board testing free of charge).
Plated holes give you all this before even considering how you actually solder a component to the PCB - though it offers advantages there too...