I think many players rarely use PEKKA, but they often use Dragon. Should I upgrade Barracks to level 10 for PEKKA?

yes

Yes

Upgrading the barracks to its maximum level is important to unleash the PEKKA in them.
Although you can see during clan wars that most of the players use dragons in their attacks, the PEKKA can become in handy with advanced ground attacks especially in town halls level 9 and 10.
PEKKA when used in combination with other ground troops like the golem and wizards can be very effective.
They can be deployed to help destroy the remaining buildings.
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Other traps does not work on PEKKA too.
Its only weakness is the hidden tesla tower for which it is the tesla’s favorite target from all other troops.