Frog is provided to increase the bong between mortar and brick and also the lateral load resistance.
Its like a key to bond.
This is provided as bricks are small in size so number of layers fixed in mortar are more.
The technology advancement seen in aac blockwork, now days thin bed mortar adhesives are used so it less likely to get affected.
If you use it with mortar a loading cracks can be seen over period of time on it.
Frog or depression in brick also used to emboss trade mark.
In aac block you can see it on sides of some blocks. Its on mould which is used for manufacturing whole block and then wire cut into small pieces.