Tamil is descended in part from Grantha and in part from Vatteluttu. Grantha and Vatteluttu are relatively closely related, and the two were used together for a time, so it was relatively easy to smear the two together.
Tamil has changed significantly as technology has changed. When Tamil was written on palm leaves, there was a tendency not to write the virama (which in the Tamil script was a dot above the character) because of the tendency of palm leaves to tear. Once Tamil started being printed, people started using the virama more regularly.
The Tamil spoken language has far fewer consonant clusters than most Indic languages, which means that there are fewer vowels that need to be killed, which means that there is less need for the virama (called pulli in Tamil). It also means that there are fewer consonant conjuncts in the Tamil script than in most Indic scripts. Almost always, the virama is used to kill the vowel. There are only a few consonant conjuncts.
Tamil was once my choice for “prettiest writing system”. I have a new favorite, but I still find Tamil to be a very pretty script.