Amro,
examples of useful trends:
What happened with expected project duration during project execution,
What happened with forecasted project total cost during project execution,
What happened with resource productivity during project execution,
What happened with probability to meet project target, etc.
Analyzing trends you may learn where and why there were problems, make better forecast of future performance, get early warning on potential problems.
Just imagine that your project is 10 days ahead of baseline schedule but one month ago it was 25 days ahead of baseline and two months ago 40 days ahead of baseline, You discover problems that are not seen if to look at the project status and will apply timely corrective actions.
Trends supply management with very useful information for deciision making.