We live in a borderless world. Many choose to study and work overseas, requiring a visa. Visas for study and work are issued for a limited time and have to be renewed, which can be quite a hassle. PR is a better option in such cases.
With PR, you can stay on in the country permanently, while retaining your own citizenship. PR holders get many rights equal to that of the citizens, except the right to vote or applying for positions under the government.
Having residency status legally grants you permission to live in the country for a specified period of time. That means you are a legal inhabitant in the country. The status of permanent resident gives you the right to stay indefinitely. That said, permanent residents have quite a few limitations compared to U.S. citizens. All permanent residents, after a certain duration of time are eligible to apply for citizenship.