A coat-of-plates, with larger plates, of the same weight, will disperse the force from blows over a larger area. This should make it more effective than a brigandine against some weapons.

But the brigandine is likely to perform better against some other weapons, especially arrows and crossbow bolts. These weapons were a serious threat on the battlefield, and keeping them out of one's delicate body was an important task for armour.

The plates on brigandine are individually more mobile. Consider a brigandine plate hit by an arrow. The plate might weigh about 16g (assuming a 5cm by 5cm by 0.8mm plate), which is less than the typical war arrow. When the arrow hits it, it will move, which takes energy from the arrow - there will be less energy available to punch a hole through the steel. This movement could rob the arrow of up to 20% of its kinetic energy. Next, the plate hits the plate behind it, which is, in turn, pushed along. More energy lost. Once something solid enough stops the plates from moving - the wearer's body - the arrow has to make a hole in 2 plates (typically, for a European brigandine, there are two layers of steel over most of the body, and 4 at a smaller part (some Asian brigandines have a lot more overlap, and present 4 layers over much of the body)). The arrow's energy will go into two things now: punching a hole through the plate, and otherwise deforming the plate. As an arrow hits the relatively thin brigandine plate, it bends it, which will rob the arrow of more energy.

While a thin plate like this is penetrated more easily than the thicker plates common on coats-of-plates (perhaps about 1.5mm), the arrow still needs to go through two of them. The plates move more, and deform more, both of which reduce the kinetic energy of the arrow available for penetration.

For some tests on arrows vs brigandine (which performed well), see chapter 5 in Hugh D. H. Soar, Secrets of the English War Bow, Westholme, 2010.

Other armours will gain improved protection against arrows due to being mobile, for example mail, and lamellar (especially if the rows aren't laced tightly together). Robbing projectiles of energy by deformed was also used on bullet-proof plate armours made of two, sometimes 3, thinner layers joined together, as described in Anthony de Reuck, David Starley, Thom Richardson & David Edge (2005) Duplex armour: an unrecognised mode of construction, Arms & Armour, 2:1, 5-26, DOI: 10.1179/aaa.2005.2.1.5

For a nice photo of a brigandine, in which the overlap giving at least two layers can be seen: https://collections.royalarmouries.org/object/rac-object-36987.html