You Do Need power plug adapter while traveling from Benin to Tonga.

Here is Why You Need An Adapter

The outlet and plug type used in Tonga is I and in Benin are C and E. This is how they look like.

Benin

In Benin, outlets of type C and E are in use.

Type C

Type E

Tonga

In Tonga, outlets of type I are in use.

Type I