Well, налево and влево are synonyms, but often they cannot be used interchangeably. For example: "машина повернула налево" (манёвр на перекрёстке) vs "машину тянет влево" (неисправно рулевое управление). We can say "машина повернула влево" and "машину тянет налево", but it sounds a little bit strange (at least, for me).

In my opinion, they differ in their meaning in the following way:

Налево denotes the change of direction: We go in some direction initially and then we turn and go налево, to the left.

Влево denotes some characteristic of direction: a direction is such that it is related to the left side.
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