The update statistics commands update the column-related statistics such as histograms and densities. So statistics need to be updated on those columns where the distribution of keys in the index changes in ways that affect the use of indexes for your queries.
Running the update statistics commands requires system resources. Like other maintenance tasks, it should be scheduled at times when load on the server is light. In particular, update statistics requires table scans or leaf-level scans of indexes, may increase I/O contention, may use the CPU to perform sorts, and uses the data and procedure caches. Use of these resources can adversely affect queries running on the server if you run update statistics at times when usage is high. In addition, some update statistics commands require shared locks, which can block updates. See “Scan types, sort requirements, and locking” for more information.