Why did Dred Scott take Emerson and Sandford to court? What did he want?

Though born a slave in a slave state, Dred Scott had been taken by his owner, Dr. John Emerson, to a number of free states and territories. Emerson was a surgeon in the US Army and, as part of his job, often had to move around the country. On one such work assignment, he ended up at Fort Snelling in the Wisconsin territory, where slavery was prohibited. By bringing a slave into a free territory, Emerson was effectively violating the Missouri Compromise of 1820.
During his time in the Wisconsin territory, Dred Scott got married, something he would not have been able to do had he remained in a slave state. Yet Scott's owners continued to hire him, and his new wife, out as slaves, despite their residing in other free territories such as Illinois. Scott wanted to buy freedom for himself and his wife, but his owners refused his request. So Scott felt he had no choice but to take the matter to court. The case went all the way up to the US Supreme Court, which in an infamous ruling, held that Scott, by virtue of his "inferior" race, was not an American citizen and as such had no rights that the Court was bound to respect.
https://www.oyez.org/cases/1850-1900/60us393