They can lead to other things.
From the BBC:
Roman coin find leads to Roman town, Devon, England
England's western-most Roman town uncovered
By Louise Ord
Assistant Producer, Digging For Britain
Roman coins found by two local men led to the discovery of a town
A chance discovery of coins has led to the bigger find of a Roman town, further west than it was previously thought Romans had settled in England.
The town was found under fields a number of miles west of Exeter, Devon.
Nearly 100 Roman coins were initially uncovered there by two amateur archaeological enthusiasts.
After the coins were unearthed by the local men out using metal detectors, Danielle Wootton, the University of Exeter's liaison officer for the Portable Antiquities Scheme (PAS), which looks after antiquities found by the public, was tasked with investigating further.
After carrying out a geophysical survey last summer, she said she was astonished to find evidence of a huge landscape, including at least 13 round-houses, quarry pits and track-ways covering at least 13 fields, the first of its kind for the county.
"This was a really exciting discovery," said Ms Wootton. But she said most exciting of all was that her team had stumbled across two burial plots that seem to be located alongside the settlement's main road.
An example of the coin which might be encountered in England:
Emperor Septimius Severus (AD 193-211) died at York.
This silver Septimius Severus Denarius commemorates a victory over Parthia (Iraq).
Obverse: Laureate head right
SEVERVS AVG PART MAX
Reverse: Victory flying left holding open wreath in both hands over round shield set on low base.
PM TRP VIII COS II PP
Struck: AD 200