Ability to use Cadence or similar tools from Mentor Graphics is an important learning objective of the integrated circuit design courses. There are plenty of tools available for free and are easy to use, but you can not reliably design, simulate and fabricate a high-performance chip using them.
Further, Cadence has integrated thousands of functionality and software pieces into their tool-set, over several years, which encompasses most of the aspects of circuit modeling, simulation and synthesis, layout design and verification, which the free tools can never match even by far. This is true for both Analog as well as Digital (also requires experience with Synopsys tools) IC design.
Lastly, most of the major Analog and Digital IC design companies specifically ask for Cadence-related experience when recruiting. There is a very high chance that you will be using these tools in your job, internships or research project and you will need prior training with these diverse tools.
Here's a snippet from the recruitment call from a major Analog/Mixed-signal chip design company: “Knowledge in Cadence tools for analog design and simulations is a must.”
PS: You can use LTSpice for your own simulations but they will not be accepted as homework/project solutions, unless and otherwise specified.