Quantitative analysis of journals is a way traditional peer review may be augmented to gain a more complete picture of a scholar's impact in his chosen field. Three measures can be used:
Publishing traditions vary between disciplines. Because of this, it is important to compare journals within the same or similar disciplines as much as possible. This will not always be easy as more research becomes interdisciplinary but in order for these journal ranking systems to have meaning the factors measured must compare journals in similar disciplines or subject areas.
Just as you may be aware of predatory journals, seeking to make money from academics eager to be published, fake impact factor sites have appeared on the web. When trying to determine where to publish try as much as possible to use well-known journal ranking resources.
What "impact" do metrics have on promotion and tenure?
Those are decisions made by each department or college on campus. The best thing to do is to consult with the department chair or colleagues to learn what is useful in your discipline. You'll need a selection of tools and the set will vary by discipline.