What is lvmetad and why would I want or need to use it?

From this link:

Normally, each LVM command issues a disk scan to find all relevant physical volumes and to read volume group metadata. However, if the metadata daemon is running and enabled, this expensive scan can be skipped ... This can save a significant amount of I/O and reduce the time required to complete LVM operations, particularly on systems with many disks.

So you would run it for increased performance of LVM management and status operations, at the cost of startup performance and increased complexity. The level of performance increase is larger when there are more disks in the system.