Sometimes ConTeXt beta releases have issues. The ConTeXt suite allows you to test the latest ConTeXt release without worrying whether upgrading will corrupt a critical project. Multiple ConTeXt suite installations are possible. This allows, for example, a stable version and a beta version to run in parallel on the same computer.
ConTeXt is also included in TeXlive. The ConTeXt suite is updated continuously, whereas ConTeXt in TeXlive is updated less often. In particular, <code>luatex</code> and <code>metapost</code> are under active development and their binaries are updated frequently. ConTeXt follows these developments, indeed often drives development. As the binaries included in the TeXlive distribution are only updated yearly, ConTeXt on TeXlive will necessarily be less current than the ConTeXt suite.
(ConTeXt on TeXlive can be kept somewhat more current using the http://tlcontrib.metatex.org repository.) The ConTeXt suite is self-contained and can happily coexist alongside another installed TeX distribution.