It would be nice to think that each type valve had its own symbol and that all valves can be clearly defined, however, the international standard has been the focus of much discussion for many years and is something of a compromise. The problem is that manufacturers often have subtle differences in the detail of their valve design which they'd all like to form part of the standard symbol.
A relief valve, for example, may always have the same basic function but can be constructed in significantly different ways. Cost may vary between £10 and £1000 and performance may be equally different. As a hydraulics engineer, it's important to understand the difference but this may not always be possible from the symbol. For example knowing where whether the valve uses a sealed poppet, spool element or both can make a big difference to the circuit operation; as can pilot feed or orifice positions etc