Engine was probably not serviced regularly.
i.e. changed oil, spark plugs and air cleaner.
These all effect efficient combustion, when it isn't as efficient you get carbon residue, that is what clogged EGR tube and valve.
EGR system is there to lower combustion chamber temperatures when engine is under load, not to reburn exhaust gases, lower temps reduces NOx emissions, side effect is engine won't ping with advanced timing on regular gas.
If you delete the EGR system your engine may ping/knock under load, on carb engines you could just run engine richer or retard timing.
On EFI engines the computer expects EGR system to be working so you may get pinging when accelerating or climbing hills, and may have to retard timing(so spark happens sooner), this reduces MPG and performance.