What was Mendel's contribution to science and what were the resulting benefits?

Gregor Mendel was an Austrian monk, botanist, and the father of genetics. He conducted a number of experiments and observational studies of the plants he tended around the monastery. His most famous discovery resulted from working with pea plants, where he noticed that traits like blossom color followed a particular pattern of inheritance. Today we call this pattern and its study "Mendelian Genetics." Mendel determined that certain traits in his pea plants were either "dominant" or "recessive," and that the recessive traits were far less likely to be expressed as compared to the dominant.
Have you ever worked with Punnett squares in science class? This type of diagram draws directly from Mendel's identification of dominant and recessive genes. By "crossing" two specimens of known genotype, you can predict the probability of all possible resulting phenotypes. 
Though Mendel made no efforts to publicize his own work, it has become indispensable to the study of genetics. He was the first to create the idea of the connection between an expressed trait and its determinant, or the phenotype and genotype. Later work by scientists like William Bateson expanded upon and revised Mendel's theories of inheritance. The combination of Mendel's work with that of Charles Darwin has laid the foundation for the modern study of genetics, biology, and evolution.
Without Mendel, there might well not be a study of genetics. Without the study of genetics, humans would lack the capability to better understand and intervene in the workings of biological inheritance. Conditions like hemophilia in humans as well as certain types of blight in plants may be addressed and remedied through genetic interventions.
https://history.nih.gov/exhibits/nirenberg/HS1_mendel.htm

https://www.britannica.com/biography/Gregor-Mendel

https://www2.palomar.edu/anthro/mendel/mendel_2.htm