Would you choose a route or altitude based on the information in a constant pressure chart? How does a constant pressure chart help a pilot plan his or her flight?
As others wrote, I don't think in and of itself the constant pressure chart will help with a go or no-go decision by itself. However, I do believe it would help to get the big picture of what's happening with the weather; fronts, etc. Knowing the big weather picture is more important depending on what kind of flying you do. The constant pressure chart can help with that. Regarding choosing a flight level or altitude to fly in I would use winds aloft, airspace charts like a sectional, anticipation of ATC given altitudes in the area you plan to fly in, etc. Like many others have said: with regards to weather the more you know the better off your preflight planning will be and the constant pressure chart adds to your knowledge of the weather.
A constant pressure chart by itself doesn't provide any useful data for flight planning.
As it shows a height of a given pressure level, you can make some assumptions about air mass temperature and pressure systems. You could also determine wind velocity and direction to some accuracy.
However most of the information available is usually shown in a pre-computed and pilot-friendly way in other charts, such as sw-charts and wind/pressure charts.