Two reasons. For one, Turkey has/had an official casus belli against Greece for Greece seeking to claim its full international border rights. This is a hangover of the dissolution of the Ottoman Empire that has been plaguing relations for the last century. This is not bad blood from the past. The two countries were headed to war in 1998 before NATO intervened over the rights to an island in the Aegean. As the Empire was falling apart, Turkey was forced to cede islands to a European alliance. Those islands, primarily under Italy's control, were later ceded to Greece after WW2. These were islands that were predominately populated by Greeks (95%) during the last century and historically. But Turkey only grudgingly ceded that territory and has never made it easy. This has been THE major problem between the two countries, and they have had constant dogfights, with hundred million dollar planes plunging into the sea, pilots dying, etc. It's constant and nasty.

Another reason this may become important is for the delimitation of the continental shelf for future oil and gas exploration. Personally, I can never see this happening given tourism and the environmental movements in Greece. But it's there.

The impact on Europe will be considerable. Greece has demanded that Turkey engage the issue at the International Court level, as a process of law. Turkey has refused, and this has hurt them. They are seen as unwilling to engage in European legal processes. Turkey isn't even a signatory of the law of the Sea, and though Turkey begrudges Greece its full buffer, Turkey takes advantage of that full buffer for itself.

The impact on NATO will be notable, as Greek and Turkish defense systems are arrayed on precisely this issue. If an agreement creates a rethinking of that defense strategy, this may also impact European Defense Forces integration with NATO, which Turkey has been preventing.

Finally, the two countries may peel back from the ungodly amounts they spend on military weaponry.

It's big news.