Alignment is key to prevent injury reduce pain and improve performance. If one builds a house on unstable foundations then the walls will move and cracks will appear. It is the same if one trains on a crooked platform, you will compensate and cause certain muscles to pull and be challenged which will potentially cause pain, discomfort, injury or a lack of performance.
If you perform shoulder press when the shoulder is misaligned and the bones are no longer articulating on the bearing surfaces that they are designed to articulate on then you will cause wear and tear, as well as potential muscle strains.
One can assess biomechanics extrinsically and intrinsically. Extrinsic or external biomechanics assessments basically compare your action to that of someone doing your sport really well – they do not take into account your ability to actually do the action in that way, they do not look at your joint make up or your muscle length or the neurology and how clear the nerve path is. It is all very well to look at the external movement but intrinsic assessing looks to see if the body is capable of moving correctly, is there an issue at the joint and if so what, in muscle length, nerve impingement? All these things come clear when one is checked over thoroughly and if the answers are not clear then there is a good referral network to go to.
See the page on assessments: Biomechaincs Assessments