Looking at the original question, the general 250kt speed limit comes from ICAO. Summarising, there is a limitation of 250kts below 3050m amsl (or 10,000 ft or FL100 depending on how the height is defined) in airspaces where there may be VFR flights and ATC does not provide IFR flights with standard separation from them. So, in airspace classes A and B, and C for IFR flights, the speed limit does not apply. ICAO is not explicit but presumably the speed limit is intended to give a fighting chance to 'see and avoid' VFR traffic.
As wellie mentioned, there are other reasons for having a speed limit such as ensuring that aircraft stay within the assumed/protected area around defined routes (SIDs and STARs in a terminal environment, for example).
In some States, the national law may also set out additional situations where the general; speed limit applies (or doesn't). No doubt there are other factors that may affect the speed limit for a particular flight or route in some cases.
I would guess that the chart makers have been asked to put the speed limit on the plate in order to overcome any question about whether the rule applies for that route.
Speaking as a controller who used to provide services in a variety of airspace classes (A, D and G around one airport), my abillity to approve a speed above 250 kts was determined by the airspace class and the height of the aircraft.