I always thought that you had to use the Developer Site Template for the site you used for debugging when building a SharePoint App. I just tried on two different machines using VS 2015, 2013 and 2012 and I was able to run and debug a SharePoint-Hosted App that targeted a Team Site in a SharePoint 2013 on-prem environment. I got an "Sideloading of apps is not enabled on this site" error when I tried to do the same thing against a Team Site in SharePoint Online. Anyone know what the difference is?
Please note, I'm not asking how I can use a Team Site instead of a Developer Site in SharePoint Online, I'm asking why there is a difference in the way Visual Studio deploys apps between SharePoint on-prem and SharePoint Online and ideally, what that difference is.