The dash has several legitimate uses, though not so many as are ascribed to it by those who treat it as a labour-saving device that spares them the trouble of thinking of the right stop. On account of this seductive property it should be employed with discretion. The following recognised usages are probably enough for all ordinary purposes, and it will be wise not to go outside them.
1. In pairs for a parenthesis.
2. To introduce a correction, amplification, or explanation of what immediately precedes it:
The National Joint Council consists of 22 members - 11 appointed by each side.
The plan is concerned with the situation some ten to fifteen years ahead- say about 1960.
3. To gather together a composite subject before passing to the predicate:
The severely crippled, the social misfit, the mentally unstable-all have found their way into the public assistance institution.
4. With a colon to introduce a substantial quotation or a list (e.g. as follows:-). This, though common, is unnecessary since either the colon or the dash can do all that is needed by itself.