To come to power, even the big parties have to join hands with the regional parties. The most recent example of this is the coalition of Congress with the Indigenous Nationalist Party of Tripura. In 2012, this party won the Assembly Elections but in 2018, no politician from this party won any seat. So it is difficult to tell why Congress has made INPT its ally. It could be the rekindling of the old alliance though.
Since a few years, Congress’s performance in elections has not been satisfactory, so it’s necessary that it coalesce with other regional parties and form a majority. It is also a good thing for these parties to come forth more aggressively and display their presence.
Keeping all this in view, the coalition doesn’t seem odd. Since INPT itself has not performed very well, we can’t be sure that Congress will win with this party by its side. It can help it win some seats, though.