Handouts help you convey the most important topics, provide a framework for note-taking, and enable your learners to refer back to something tangible when they go to use the skills or knowledge you are teaching them. It helps enhance both retention and application of the skills, therefore increasing performance.
Before you deliver the class, after you have determined your objectives and activities, create your handouts.
Handouts should not simply be a printout of your slides. However, that is a great start, and it is better than nothing! I encourage you to go beyond these printouts to think about creating something that will help your learners organize and synthesize information. If you are presenting a complicated diagram or chart, reproduce it larger for your students. They could add labels as you talk about each part, for example. Think about how your learners will be using the knowledge you're teaching them--make it easy for them to reference this information later.
The handout could be printed out or in electronic form. I like to save trees as much (or more!) as the next person, but I also value having a framework in which to take notes, organize thoughts, and refer back to later. Sometimes, I even hang the handout next to my desk (currently I have a chart of Bloom's verbs, a performance analysis flow diagram, and a Human Performance Technology model hanging around my workspace. Looking around, several of my colleagues have Bloom's verb charts taped to their monitors, too. Above my desk are manuals, references, and textbooks, some of them from trainings I have attended. I love being able to page through the manuals to remind me of an activity we did in class or the 5-point framework that was discussed, for example. Providing this as an electronic reference could also work; that way, people could decide whether or not to print it out to address their own needs. If you decide to do a solely electronic version, please consider sending it to your participants in advance so that they can decide if they should print it out. Several times I have been in classes when I have heard, "Oh yes, we'll giving you this handout at the end, you don't need to take notes," only to be disappointed by the lack of detail in the handout afterwards.
Now that you have created your handout, do you really need to hold the training or class after all? Can you shift the focus of your class to teach people how to use the handout in their jobs or real life, or a discussion about the content? Can you shorten the length of your class?
I cannot think of a single time when I attended a professional training class without a handout. However, the informal training classes within our company, as well as the community/ recreational classes I have attended, frequently lack handouts. It is my professional opinion that every single class and training include a handout of some sort. [Note: college classes may be different. Higher education is not my area of expertise!]