According to Islam, human sexuality and all of it expressions are meant to serve exclusively one sacred purpose: family life and its essential functions such as reproduction. Therefore Muslim men and women are required to avoid any situation or action that might possibly funnel any of the physical or physiological expressions of their sexuality into areas outside married life. That's why women are enjoined to cover their body outside home and act modestly to reduce to zero the chance of sexually attracting stranger men during their social interactions. This guarantees marital loyalty and prevents human sexuality to operate in ways other than its fore-mentioned sacred legitimate function.
The reason why this practice seems strange nowadays to most is because of the dominant secular culture of our time. Indeed nowadays the modern man has declined spiritually and morally so low to be not only blind to the sacred foundations of our nature and practical implications thereof, but has even perverted into normalizing and legalizing such obscene practices as nudity, homosexuality and pornography as in the so called Sexual Revolution of the 1970s.