The Suez Canal does two things that have historically been regarded as being of strategic importance and a third that's somewhere between tactical and strategic.
It cuts the length of time to get from Europe to India (and points further east adjusted accordingly) roughly in half. The British came to view this, along with Gibraltar, as vital to maintaining supplies and communications with the empire. The canal was initially joint-owned by the British, French and Egyptians but Egypt was forced to sell its shares giving the British overall control. Ironically the British were not overly enthusiastic about the canal's initial construction but within a matter of years it was considered of critical strategic importance. This declined, in turn, with the loss of Britain's imperial possessions and general withdrawal from "east of Suez" after World War II.
It's a huge source of revenue for whomever controls it. The tolls imposed on merchant shipping travelling through the canal provided huge revenues to the Suez Canal company and today to the Egyptian government. The initial revenues from seizing the canal were used to finance the Aswan dam and the canal remains one of the largest sources of revenue for the Egyptian government.
As a major water crossing it was a major barrier in the Egyptian/Israeli wars in Sinai. In 1967 the canal was a natural place to stop the Israeli advance, while up to 1973 the Bar Lev line was built as a series of defensive strong points for Israel to defend occupied Sinai. The Egyptian crossing of the canal, breaking of the Bar Lev line and temporary control of both banks of the canal is to this day regarded as a major victory by the Egyptians in the '73 war.