[aidenn0]

It's only illegal if you get net gain out of it[1]; if you turn down the power so that the maximum power transmitted in any direction is the same as before, then it's legal. You can get a small performance improvement by using directional antennas in this way as you reduce the received strength of off-axis signals which can reduce both multipath interference as well as interference from other stations.

It's not all that useful indoors though as 2.4 and 5GHz signals bounce off of walls really well so you lose much the directionality at the very first wall it hits.

1: Actually there is a total ERP requirement, but most routers with external antennas are already close to that requirement, so more than about 3dB gain will often put you over.