With most of the UK rivers levels running severely low at the moment its always useful to detect any change that localised rain may have in raising them up a little.
Also on the flip side of the coin when we start to get some of the deluges we are more accustomed to its worth a quick check on this site to make sure the river you've got lined up to fish is not going to be bank high and unfishable..it may well save you a wasted journey!
This is a site I use quite a bit for up to the minute river levels for England and Wales.
Environment Agency - River and sea levels