The rubber valves lets you put air into the otherwise fully sealed tyre and wheel. This rubber valve stem can perish and break causing a sudden deflation of air from your tyres. Inside the valve you have an inner valve core which when depressed allows the air to pass; this valve core has a small rubber o ring which also can perish and be a cause for air loss. To prevent this ensure that you have your valve dust cap fitted with a rubber o ring in the top to create a seal. When having new tyres fitted it is best for a new valve to be fitted, as usually the valve has been on the wheel for 1-2 years and maybe more. If for some reason that you have to change your tyre before it has worn out, for instance a puncture which is unrepairableor your changing your tyre size, then a new valve might not necessarily be needed.
Always check your valve condition when you do your weekly or monthly tyre pressure checks, this can only be and external check unless you have the right tool, but as long as the valve cap is fitted the inner valve core will not suffer any damage.