The word Asana is derived from the sanskrit root word 'as' which apparently means 'to be' and so, the word asana can be construed to mean 'to be with the body'. I am amazed at the erudition that was needed for the creation of this word.
Yoga sutras of Patanjali, the most famous and ancient reference text for yoga, defines yoga as 'Yogaś citta-vritti-nirodhaḥ''. This means, yoga is the cessation of the fluctuations of the mind. Fluctuations of the mind refers to the minds uncontrollable requirement to think. Yes, it thinks when we want it to, but it thinks when we do not want it to also (try switching it off for two minutes). For the average person, it really stops only in deep sleep. Thus, yoga is the stopping of this involuntary fluctuations of the mind.
To accomplish this, masters around the world have suggested different techniques. Developing concentration is one of them. The technique of asana is thus, 'to be with the body'. When we place our body parts in slightly unnatural positions and focus on the breath, it enhances our concentration manifold and moves us a bit closer to 'citta-vritti-nirodhaḥ'', cessation of the fluctuations of the mind!