Siksa sastra may be said to be a "guard" to ensure the right enunciation of a (Vedic) mantra. But it is Chandas that determines whether the form of the mantra is right. Of course the form of a mantra can never be wrong. The mantras, as mentioned so often, were not created by the sages and are not the product of their thinking. It was Bhagavan who caused them to be revealed to them. Man, beast, tree and other sentient creatures and insentient objects of creation exist as they should be according to the law of nature. In the same way, the metre of a Vedic mantra must be naturally correct. However, Chandas helps us to find out whether a mantra or sukta that is being taught or chanted has come down to us in its true form. We may check the hymn according to its metre and if we find it faulty we may correct it in consultation with people who are well-versed in such matters.
Apart from the mantras, which appeared on their own, are the composition of poets. Chandas is of help in giving shape to poetic thought and imagination. Like tala to music is chandas to poetry.
It is because poetry is composed according to a certain measure and its rhythm determined in a certain order of syllables that it acquires a definite form. It is also easy to memorise. Modern society is discarding all those rules of discipline meant to give it a definite character and purpose. In keeping with this new trend, poetry too is being written without any metre and "poets" compose as they please. People don't realise that to be free means to be firmly attached to a system, that discipline is the road to a higher freedom.
Chandas is the means by which we ensure that the Vedic mantra is preserved in its original form, it being impossible to add one letter to it or take away another. The very purpose of the Vedas is the raising up of the Self. Must we then permit a single sound to be added to it or be taken away?
For a general background, please see here