yo yo yaam yaam tanum bhaktaha shraddhayaarchitumicchati |
tasya tasyaachalaam shraddhaam taameva vidadhaamyaham || 21 ||

Whichever devotee desires to faithfully worship a particular form, I fortify his faith in exactly that (form).
yaha, yaha : whosoever
yaam, yaam : whichever
tanum : form
bhaktaha : devotee
shraddhaya : faithfully
architum : worship
icchati : desires
tasya,tasya : that
achalaam : firm
shraddhaam : faith
taam : in that
eva : exactly
vidadhaami : make
aham : I
The chapter so far dealt with the notion of the infinite Ishvara’s ability to bless us with infinitude. But Shri Krishna recognized that not all devotees have the vision of pursuing that infinite Ishvara. That is why they worship finite, localized forms of Ishvara in the form of deities such as Ganesha, Saraswati and so on.
So in this shloka, Shri Krishna offers a ray of hope for such devotees. Even if we worship a finite deity with a finite goal, Ishvara will ensure that our finite goals are awarded to us. In doing so, Ishvara will strengthen our faith in that deity.
There is nothing wrong in asking for something finite. All notions of spirituality, no matter how finite the goal, are equally valid. They have the power to lift us from our egoic centre towards the universal.
For example, if we ask Saraswati for success in our exams, it is Ishvara who delivers that success through Saraswati. Saraswati becomes the channel through which Ishvara works. He is the running the whole show, as it were. But more important than the delivery of our desire is faith. Ishvara ensures that our faith in Saraswati is strengthened. It is because there will come a when that faith will be eventually turned towards Ishvara. More about this faith is mentioned next.