Marx was drawn to the labor theory because he believed human labor was the only common characteristic shared by all goods and services exchanged on the market. For Marx, however, it was not enough for two goods to have an equivalent amount of labor; instead, the two goods must have the same amount of "socially necessary" labor.
Marx used the labor theory to launch a critique against free-market classical economists in the tradition of Adam Smith. If, he asked, all goods and services in a capitalist system are sold at prices that reflect their true value, and all values are measured in labor hours, how can capitalists ever enjoy profits unless they pay their workers less than the real value of their labor? It was on this basis that Marx developed the exploitation theory of capitalism.
https://www.investopedia.com/terms/l/labor-theory-of-value.asp
Marx used the labor theory to launch a critique against free-market classical economists in the tradition of Adam Smith. If, he asked, all goods and services in a capitalist system are sold at prices that reflect their true value, and all values are measured in labor hours, how can capitalists ever enjoy profits unless they pay their workers less than the real value of their labor? It was on this basis that Marx developed the exploitation theory of capitalism.
https://www.investopedia.com/terms/l/labor-theory-of-value.asp