Why did Vic Fontaine sang entire songs in the later seasons of DS9?

For those who watched DS9 during its first airings, did it seem odd to you that Vic Fontaine/James Darren sang entire songs in the later seasons of DS9?

I only finished watching DS9 recently and just found it really odd. It seems out of place in the regular TNG/DS9 format, didn't drove the plotline forward, and sometimes felt just like a filler.

(I don't mean to be disrespectful, I like the character and the actor can sing well, I am just curious why the producers made that decision).