Sediment accumulates in the bottom of a storage hot water tank over its working life. It settles out of the water and stays there, and it has practical consequences.
The first is capacity. Sediment occupies volume that would otherwise hold hot water, so a tank that once met demand comfortably starts running short earlier than it used to.
The second is efficiency. On an electric system the element sits within the tank, and sediment settling around it insulates it from the water it is meant to be heating. The element works harder for the same result, and it fails sooner.
The third is noise. Rumbling or popping from a storage unit often means water is trapped beneath a sediment layer and boiling there, which is worth investigating rather than ignoring.
Scale can also affect tap aerators, showerheads and appliance inlets. In those cases the symptom is reduced flow at one outlet while everything else is unaffected, and cleaning or replacing the aerator usually resolves it.
If a hot water system is leaking, making unusual noises, or delivering water hot enough to scald, switch it off at the isolation point if you can do so safely and call a licensed plumber. Tempering valves and pressure relief valves are not do it yourself items, and a failed relief valve can be dangerous.