A bottom bracket is under a great amount of pressure and force, this component assists in transmitting your power into forward movement. Bottom brackets have changed over the years evolving and become more efficient for transferring power from the body to the bike. As well as longer lasting due to sealed bearing technology.
Choosing a bottom bracket is based upon your chainset and frame type. External bearings and pressfit bearings are becoming standard fitment on bicycles. Generally, the higher up a brand hierarchy you go, the better the bearings, sealing and material quality will become. Sealed bearing bottom brackets are pretty much standard these days due to the maintenance free benefits.