See https://en.wikipedia.org/wiki/Portolan_chart for a discussion. The basic idea is that you want to be able to navigate using a compass, a chart, and some basic tools like a divider for marking distance and a tool for extending a line (or making parallel lines). A rhumb line is a line of constant compass bearing; on a Mercator projection, it is conveniently a straight line. The networks of rhumb lines that you often see on old charts are conveniences for navigation.

One way to use such a compass rose element is to place the center near or on an area that you commonly navigate from. Then you can get out a string and some tacks to plot a course and follow your compass to the destination (assuming an appropriate chart projection and navigation area). Another way you're likely to see them is as a set of 8 or so roses around the edge of the chart (often pre-printed). During exploration of an uncharted area via ship, it is then "easy" to use a compass and distance to map out the coastlines. The multiple roses make it quicker and less error-prone to mark out the correct angles. Navigational aids such as a sextant make finding latitude simple enough for correction, but it took quite a while to get accurate enough clocks to work out longitude for navigational purposes.

As Falconius points out, most folks here don't use rhumb line networks for anything more technical than decoration.