For the same reason ANY other major power uses them for: BLACKMAIL, THREATS, and as a COVER-SHIELD to do other "dirty little things", because no-one dares to pick on any country with nukes.
See: North Korea, Pakistan, France, Israel, UK... not to name the biggest ones (which we all know).
Turkey could grow and become a powerful regional force in the Middle East/West Asia with effect on Central Asia as well.
There are some speculations that Turkey wants to remove itself from Nato and create a "Turkic Nato" or a "Central Asian Nato" by attracting a number of other nearby states in.
Nevertheless: watch how no-one dares to pick on Turkey (e.g. the US). Just like no-one dared to pick on Pakistan or India, when they effectuated their first nuke tests.