who is the spirit in the doll, I guess it's not Valak?
This was some demonic presence that wanted a soul it could inhibit. First, it tried to take the soul of Mrs. Mullins, but didn't succeed and later it tried for Janice.
From the story told by Mrs. Mullins,
It started small, the contact.
But it was her... our busy Bee.
We knew it was her.
She wanted permission
to move into the doll...
...so she could be with us forever.
We said "yes,"
and that's when it became stronger.
Soon, we could actually see her.
Only in fleeting glimpses, but it was enough.
Our beloved Bee...
Even though I couldn't hold her...
...it was comforting just to have
the buzz of activity again.
But we soon realized
it wasn't our Annabelle at all.
It was a demonic presence,
and it was using our Annabelle...
...to manipulate us into giving it
a soul it could inhabit.
It tried to take mine...
...but Samuel reached out to the Church.
They saved my soul...
...but the evil
was still attached to that doll.
So we locked it away...
...where it could be surrounded
by the word of God.
That doll has been a conduit used by demons for achieving desired goals. So, whatever, the demonic presence present in that doll is after a soul it can inhibit. So, it's not Valak.
the evil spirit in the doll Annabelle got interested in terms of finding human hosts, Valak helped it in doing that but why?
Both are demons, so maybe that's why Valak helped the demon ;-). However, this is pure speculation. Though, it was expected the purpose of Valak to be explained in The Nun but it didn't do so.
However, for now, the presence of Valak in Annabelle: Creation sets the premise for The Nun movie.
From Screen Rant Article with emphasis of mine,
The post-credits scene ties things up by giving us a proper, direct tease of The Nun. A single, static shot, it shows a candle-lit hallway at the Abbey of St Carta in 1952. The lights down the corridor slowly begin to be mysteriously blown out before the ghostly visage of the Nun’s face appears from the shadows.
We already knew the film was going to be set in Romania, but this gives us the specific location and date. Carta Monastery is a real-life church, again allowing the series to play with a faux real horror angle, although it’s the year that’s most interested; Creation is set in 1957, only five years after The Nun. Given the pre-existing connection through Sister Charlotte, some active link is to be expected.
Though, none of the sisters mentioned in Annabelle: Creation appear in The Nun.