becoz without friction we cant walk , write on paper and many more things we cant do without friction.. in presence of friction we r able to walk , Run , write etc without it we all will not able to walk properly thats why its necessay and it is evil also becoz to over come friction we need to do work externally and on roads there is wearing and tearing of tyres and due to friction old machine r not able to give their performance best and due to it only nuts of machines also get loose thats why friction is a necessary evil