Yoga means union of atma and paramatma. It's sole purpose is about spiritual. Yoga also helps in curing some disease and brings mental peace. Can I use yoga only for this reason? I mean is it selfish act?
You are absolutely right about yoga being the union of atma with paramatma. That is yoga's real meaning.
According to Gautamiya Tantra :
Yoga is the method of going beyond the samsara. The yoga experts call the union of jivatma & paramatma as yoga.
For the Shaivites :
The knowledge of non-difference between Shiva & atma is Yoga.
For the Tantriks :
The knowledge pertaining to Shiva + Shakti is yoga.
But the thing is that, with a diseased body such an union is not possible. So, in the process of making one's atma fit for the union one also has to make the mind and body disease free.
And, the process of Yoga does both.
For example the Yoga Kundalini Upanishad says :
3-4. I shall explain the nature of these. Listen to it, O Gautama. One should take a sweet and nutritious food, leaving a fourth (of his stomach) unfilled, in order to please Shiva (the patron of Yogins). This is called moderate food. Posture herein required is of two kinds, Padma and Vajra. 5. Placing the two heels over the two opposite thighs (respectively) is the Padma (posture) which is the destroyer of all sins.
So, the padmasana, which cures several ailments, also burns away our sins at the same time. So, yoga serves a two-fold purpose.
Regarding the importance of having a healthy body in mokshasadhana this is what LOrd Shiva says in the KulArnava Tantram:
VinA Dehen KashyApi PurushArtho Na Vidhyate |........... Raksheth SarvAtmanAtmAnam....
Without the physical body the four purusharthas can't be achieved..... Hence, one should preserve one's body with care...
KulArnava Tantram, Chapter 1.
Similarly the Gandharva Tantram says:
Sariram Tu ManushyAnAm PurushArthaikasAdhanam (34|15).
Human body is the only means of achieving the purusharthas.
The four purusharthas are : Dharma, Artha, Kama and Moksha.
And, yoga serves the purpose of keeping our body and mind healthy and thereby accelerates the process of the atma-paramatma union.
One of the most popular quotes of Swami Vivekananda also has a similar purport:
YOu will be nearer to Heaven through football than through the study of the Gita." — that is one of the most popular quotes of Swami Vivekananda.
So, practicing yoga with the aim of achieving a healthy body is not selfishness . On the contrary not doing so is foolishness.