A dash preceded by a colon is sometimes used before a long quotation forming a new paragraph.

In other cases no point need accompany the dash.
The dash is sometimes used as a substitute for commas.

Writers on the subject say that this use occurs when the connection between the parenthetical clause and the context is closer than would be indicated by commas.

The distinction, if real, is difficult to see.

It would be better if none but the most experienced writers attempted the use of the dash in this way.
Dashes are often used instead of marks of parenthesis.