The answer is, it depends. As far as simply drawing stuff, you can do everything in Free that you can do in Pro. It's what you want to do after you've got the model drawn where pro comes in. Show me what you would send to a client to get their business.
With pro you'll get LayOut. This is the documentation part of the package. With it you can create a professional looking package and make a PDF file that you can then send off to the client. You can put in SketchUp views as well as other drawings, photographs, text, etc. If you need to send off dimensioned drawings or even make them for yourself, you can do nicer dimensions in LayOut than you can in SketchUp. You can quickly and easily make full size patterns for your own use if you need them. You can make accurately scaled views if that's needed.
Without LayOut you could export views of your model as images from SketchUp. Then you could insert those images into a Word document and add text and other content. You'd need to do your dimensioning in SketchUp of course. And then, if you have a PDF writer of some sort you can make a PDF file to e-mail to your client. When they ask for revisions, you will modify the SketchUp model, re-export the images, reinsert them into Word and make the new PDF. With LayOut you would edit the sketchUp model and then automatically update the views in LO and re-export the PDF. It would be much faster to just use LayOut.
Also, if you need the option to import or export CAD files, pro does those things while the free version doesn't.
FWIW, these are some screen grabs from a LayOut project. The quality isn't that great in these views but they were handy in my Flickr album. It's only two of a number of pages
And here's part of a page from another LO project.
the dashed lines and all the callouts and other text are done in LayOut instead of SketchUp.