Ga is the voiced member of the velar class, made where ka is made with the vocal cords engaged, and it sits third in its group. The placement is the useful thing about it: within every group of five the first two letters are voiceless and the next two are voiced, so a reader who knows the point of articulation and the position knows the sound without having to remember the letter individually.
The sound is common and the letter is met constantly, including in a large number of conjuncts, where it joins freely with the nasals, the semivowels and the sibilants. Its cluster with the palatal nasal, and its cluster with the semivowel ra, are among the shapes a learner of the script acquires as units rather than deriving from their parts.
The regularity the letter illustrates is what made the Indic table valuable outside India. Scholars in China and Japan studied the arrangement as an analysis of speech rather than only as a way of writing Sanskrit, because it demonstrated that the sounds of a language can be laid out as a grid of features and positions. Nothing in the writing systems they already used made that claim, and nothing in them could have.
Its cluster with the velar nasal that closes its own class is the ordinary way that nasal is met, since the nasal rarely stands alone, so the two letters are learned together in practice even though they sit two places apart in the table. Pairings of that kind are one of the reasons a chart of conjuncts is taught alongside the chart of letters.