There are a few worthwhile things about a slackbuild; its considered old fashioned , but by first by going to slackbuilds.org you can read about the dependencies and features of the deps. So you can actually learn a bit about software.
By reading up on slackbuilds it will tell you that you can, at build time, have small tweaks such as if i remember correctly the mirrors , when you run freshclam for clamav.
Actually clamav is a good example ; before you run or install clamav you need to set up appropriate groups needed. The info on the slackbuild page for clam will tell you that and how to do it.
Also when you run a slackbuild on your machine there are a couple of things that it takes account of and makes you take account of. It will take into account to a degree the Architecture of your machine. I did write a bit on slackbuilds here :
So what it means in practice is that sometimes a ready to install package for Slackware may install Ok but not work , because prep work was not done.
So slackbuilds.org is the first place to go to read up. If you then find that there are no particular prep work to do, then you can automate install of package and its deps using : https://slackbuilds.org/repository/14.2/system/slpkg/?search=slpkg
For slpkg ,you have to run its slackbuild to create a package and install it using : installpkg <packageName>
Once you've done that you need to do some editing to configure it. After that you can search and install packages more or less like any other command line package installer , using that software