Your question  is about frogs dissection. Not some
animal dissection. And you add: By scientists. Not by
other kind of person.
See you did not asked a broad question, but a single
one. And you are a student, making the question still
less broad.
In Webster we can find:
Dissection: Anything dissected as a plant or animal
for study.
Now that gives a point and a reason:
To study the external and internal parts of the frog.
And if you open the question you will have the same
answer for the dissection of any animal
(or plant) that was once alive, and by a responsable
person that wants to learn or to teach.

Why a frog? Some reasons: It is small, it is easily
found, it is not cute or used as a pet, have about
the same organs as a man´s body... And there are
one reason that can be interesting: One  dead frog leg
was used by the physicist Galvanni in a famous
electricity  experiment
(http://experimentexperimen//www.aps.org/publications/apsnews/200603/history.cfm)

and can be done by students to show also how some
bona fide experiment conclusions can be wrong and
still the experiment lead to  a different  discovery
(Volta battery).

So...is that answer enough? I dont think so. The frog
dissection have been made all over the schools in US
and other countries. By moral reasons some students
or their parents are against it. It was at least one
demand in Court because an A student got a C for
refusing to perform the task.

Nowadays there can be seen at Internet hundreds (or
more)
very well designed virtual dissections not only of
frogs,
but from all kind of animals, even big mammals.
One can interact, and learn by him(her)self.

By other hand in that field one must do with the hands
(and brain,and sensibility) to learn, in Medicine
schools one learn also  doing authopsies.

Observation: This answer shows only my opinion, and
for sure everything i said above applies to dead
bodies,not to vivisection.