Pranayama stops wastage of energy and increases stamina

Verse 3.7

त्रयमन्तरन्गं पूर्वेभ्यः ॥७॥

trayam-antarangaṁ pūrvebhyaḥ ॥7॥

These three (dharana, dhyana and samadhi) aspects of yoga are internal, compared to the former five.

Verse 3.8

तदपि बहिरङ्गं निर्बीजस्य ॥८॥

tadapi bahiraṅgaṁ nirbījasya ॥8॥

Similarly, samyama is external when compared to seedless (nirbija) samadhi

Compared to the former five aspects of yoga, it may be seen that dharana, dhyana and samadhi are more subtle, internal, intimate and subjective practices.

The first five, which deal with the seen or cognizable sheaths, are called the external quest. 

-Yama purifies the organs of action; 

- Niyama, the senses of perception; 

- Asana cleanses the physical and organic aspects of the body; 

- Pranayama stops wastage of energy and increases stamina; 

- Pratyahara cleanses the mind.

More intimately, dharana, develops and sharpens intelligence, dhyana purifies consciousness and samadhi leads mind towards the Atman. 

These three are directly involved in the subtle sheaths of mind, intelligence and consciousness and are very close to the spiritual heart. They directly affect the spiritual path, and are therefore called the inner quest, or sabija samadhi, because the sadhaka now has one-pointed consciousness.

In samadhi pada, Patanjali explained that truth-bearing wisdom (rtambhara prajna) is the threshold between sabija and nirbija samadhis.

Here he describes samyama as the penultimate step towards nirbija samadhi.

Even this perfection of dharana, dhyana and samadhi appears external to one who has experienced the seedless samadhi, the direct vision of the soul.

Sutras 3.7-8 describe the distinction between sabija and nirbija samadhi.

Surra 3.7 explains that the conquest of the vehicles of nature and of nature itself is of the foremost importance in opening the gates of kaivalya. 

In 3.8 verse it is said that as samyama is dependent on a support or a form, it is called 'external' compared to nirbija samadhi. Once the vehicles of nature (body, organs of action, senses of perception, mind, intelligence, reason) cease to function, the soul (Atman) shines forth, and the sadhaka dwells in kaivalya and not on its threshold.

Sleep comes naturally when mental activities cease without effort. In the same way, perfection in sabija samadhi takes one towards the seedless state of samadhi or kaivalya, as effortlessly as falling asleep.

PATANJALI YOGA SUTRAS