These things are steeped in history. A foot is the length of a foot. A roman pace was the average stride of a roman soldier - about a yard, but a yard was the length of a mans arm ...or a drawn arrow. an inch was a knuckle. Before measures were standardised these were handy ways to sell cloth etc.
An ell was like a cubit, elbow to fingertip.
A rod pole or perch was designed to be part of a system of surveying units of which the the chain and the mile (originally Mille Pace - 1000 paces, but now more like 2000!) were also part. A cricket pitch is a chain long.
"A chain is a unit of length that measures 66 feet, 22 yards, 100 links,or 4 rods (20.1168 m). There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains (that is, an area of one chain by one furlong). The chain has been used for several centuries in England and in some other countries influenced by English practice. In Scotland a chain was 74 feet."
These are historic measures that grew up out of necessity. They may be obsolete today in the days of laser range finders and cheap vernier callipers, but they had their uses.