Reports and charts are one of the main features the software has that the online trees don't. The online trees offer a family group sheet and a print version of each individual profile but that's about it, I think. The software offers dozens of customizable charts and reports.
Also, the software allows you to do custom filters and searches. For example, one of the features I use the most is to filter only individuals who have data for a burial so I can see a list of all the cemeteries I need to visit. I can also create a custom report with this list so I can print it out and take it with me. I can also do the reverse and view everyone who has no burial data so I can see whose gravestones I still need to tract down. You can use this feature for any fact - I've also used it for religion.