Both yagi and omni are outside antennas that pull in your existing 3G & 4G signals before sending it off to the amplifier to be boosted.
In general, omni antennas are all-around performers used in corporate offices, because they pull signal from a 360-degree field, which usually helps when boosting multiple carriers with cell towers in different locations. They generally are long rod-like cylinders.
Directional (yagi) antennas are specialized performers that pull in signal from a 45-90 degree directional field. The ability to focus on a narrower field allows it to reach farther than the omni and pull in more signal. However, unless all carriers are within that directional field, the yagi antenna tends to only boost one carrier.They generally look like an arrow tip or a pirate flag (ahoy, mateys!).
So, directional antennas are very popular with people in remote, rural areas or any place with terrible reception.