Fortress Busting: against extremely hard targets, every bit of extra firepower helps.

Defending: the movement penalty doesn't hurt when using a static defense (e.g.

Marines: since you are limited to three marine stacks in the early game, every extra bit of firepower helps.

Infantry + arty is good for defense but makes these divisions too slow for attacking through several provinces. Use either SPA or AC brigade for your mobile formations in a combined arms formation of armor and motor, the AC brigade on motor infantry divisions helps them to keep enough organization. The general rule would be to brigade expensive land divisions such as armor and motor and mech because brigades are comparatively more cost effective than expensive divisions are while not brigading cheap units like infantry divisions.

Mechanics: Some brigades have absolutely no practical value in-game under any reasonable circumstances.

TC load: Brigades use a relatively high amount of supplies and fuel, thus disproportionately increasing your TC load.

IC cost: The IC cost is too high when compared to the improvements offered: the player is better served building additional units, rather than brigades.

Stats: Improvement in fighting ability is typically minor or of limited practical utility.

The general view is that Brigades are largely useless because:

Hearts of iron 4 best division