I've seen and heard this asked many times, and I'm hoping that this will help to answer any future questions surrounding this question. Wait steps and Evaluation time period on Campaign Canvas serve 2 very different purposes. While you can often use a Wait step in place of an Evaluation time period, it's not always appropriate.
Evaluation Time Period
This is the period of time for which you want to evaluate a contact who has not yet met the criteria of a decision. When a contact meets the "Yes" criteria of the decision, s/he will immediately flow through the decision step and into the next step on the canvas. When a contact is evaluated as a "No", the Evaluation time period can hold them in this step to re-evaluate the criteria, and eventually pass them down the "Yes" path once that criteria is met. If the criteria is not met within the evaluation period, that contact will flow down the no path once the Evaluation time period has expired.
Here's the scenario. You send an email, and you want to evaluate whether or not a contact has clicked on that email. Passing the contact from your email step directly into a "Clicked Email?" decision without an Evaluation time period will cause all of your contacts to flow down the "No" path. This is because no one could possibly click the email in the seconds between when the email is sent, and when the contact hits the decision step. You provide an Evaluation time period, allowing time for your contacts to receive and, hopefully, click your message. You could use a wait step before the decision, but you are effectively holding up all of your "Yes" contacts, and preventing them from flowing through your campaign.
A complete hold on all contacts who enter this step. A Wait step will hold every contact who enters this step for the period of time set. Each contact who enters this step will Wait for the set time, regardless of when they enter.
The scenario. You have a Wait step set for 1 week. A contact who enters this step today, Thursday, will leave this step at the same time next Thursday. A contact who enters this same step tomorrow, will not leave this step until next Friday. The wait period is applied to the contact at the exact time they enter this step.
Still have questions? Feel free to post them here, I'd be happy to explain further or provide additional scenarios.