Why would I need to update stats manually?
As the data in a table changes the statistics objects that represent the distribution of data become outdated. This is a problem because it's the statistics objects that the optimiser uses to make it's estimates when building execution plans. If these estimates are inaccurate your performance will suffer.

SQL Server does automatically update statistics objects when the amount of data in the table that has been changed hits certain thresholds, but if you have data that is changing frequently or tables with millions of rows you will better off performing regular index maintenance.

This can be done with maintenance plans or scripts. Ola Hallengren's suite of scripts is very good for this and something that I use in all of my environments. There is also the Minionware suite that I have not used personally but I hear good things.