Bigger Radius is the first upgrade of Path 1 for the Monkey Village in Bloons TD 6. It gives the Monkey Village a larger influence radius, from 40 range to 48 range. This upgrade is useful for expanding the area where the Monkey Village can cover, saving money overall compared to purchasing another Village.
It is generally more wise to pre-plan Monkey Village influence range beforehand, but in the uncommon case where there is not enough influence range for the Monkey Village to grant its various buffs to nearby towers, the Bigger Radius upgrade comes in very handy. Additionally, the next upgrade Jungle Drums is a very helpful upgrade in general, which is more likely the main reason for going for the Bigger Radius upgrade.
If more influence range for the Village is needed, this upgrade is the one to buy. Bear in mind that it locks out either one of the other paths, so choose the Village's next upgrades carefully.
The next upgrade after Bigger Radius is Jungle Drums, a valuable additional support by increasing attack speed of towers within range.
There is simply no need to buy this upgrade unless there is no more space to cover more towers within range or if there is significant reason to expand the coverage range to let another tower reach a bit further away from the Village.
No changes were made to Bigger Radius since it was first released in BTD6.
1-0-2 Village crosspath, with red entrance bars referring to Bigger Radius crosspath
Crosspathing in BTD6 (Top: Path 2, Bottom: Path 3)