Grass seed, as opposed to sod, has to be planted during a specific time of year, is more vulnerable to weeds, and requires considerably more maintenance.
While sod costs more than seeds and is a little more labor intensive, the fact that it can be immediately installed regardless of the season more than makes up for its drawbacks.
You can read more about the pros and cons of sod and seed in our blog, Grass Seed vs Sod.