Hi @Doug1 
Many software editors like teamviewer or slack works this way.
Thy want to avoid maintaining one ppa that will allow you to install software only with apt install.
The benefit of installing software from ppa (apt install xxx) is that the apt software will check all packages and library the package you want to install needs to work and install them for you : (look here, I'm about to install kdenlive, and finally, many packages are required kdenlive to work )
If one of those required packages have also some dependency, the apt software will also install them. It will resolve all dependency for all packages that will be finally installed.
This way, after a install over ppa, your software will be fully working
The difference with installing .deb files is, that you will install only the target software, without installing some dependency it have. The behavior is wanted by the software editor.
After installing such .deb you need to ask the apt command for some help and help you resolving dependency and install them on your computer.
After doing sudo dpkg -i ./slack.deb you will get errors, that some dependency are missing.
Just after dpkg done the installation, run sudo apt-get install --fix-broken to finish installing your software.
The errors should get corrected and a regular sudo apt update should work without printing errors about missing software.
Sometimes, the .deb package have dependency that do not exists anymore in ppas, can be observed in old .deb files - I got something like this with IBM Sametime. If apt cannot fix the dependency, the user need to dig the web finding manually packages, that may require other packages, ... that become painfull xD Hopefully, this is not something we are facing today
Welcome on linux ! 
!
