The #1 reason people choose a gated community is the perception of increased security, due to the gate and the privacy  of the residents. Obviously, gated communities are more difficult to access than a standard community and therefore criminal activity is generally reduced.
Less Traffic
Most often, heavy traffic and speeding are reduced within the gates, creating a quieter and arguably safer community.
Higher Home Build quality
Most communities have strict building codes promoting uniformity in design. Often, this leads to a higher pride in ownership, and as driving around these communities will show you, yards and homes are most often extremely well tended.
Social
Sometimes there is a club-house or community center that promote group activities and residents getting to know each other.
More Expensive Home Prices and HOA fees
Gates and amenities come with a Home Owners Association or HOA fee. Many roads in gated communities are considered private, so the residents pay for road maintenance, usually through the HOA fee. On the other side, generally you won’t have to live in an area with holes in the road.
Your visitors and contractors will have to wait to get past the gate, and HOAs may limit contractor access times, and block them on certain days.