Vias, usually small holes, are drilled quickly. They connect all the layers and are plated in a single mass operation, so the incremental cost is very low. They are usually quite reliable and have a small resistance for slow circuits.
The reasons for avoiding vias are possible
- Impedance discontinuities
- Several milliohms of resistance for large current
- another good solution, such as using vertical trace on one layer and horizontal on another,
- Not applicable to critical high-speed controlled impedance paths because they run first.
- Less reliable than a straight trace
Certainly, if you want to use a multilayer board – 4 layers or more, they are the ticket to using all the layers effectively. You can’t do without them.