What is the need for convolution in digital signal processing?

If we need to add two signals in time domain, we perform convolution. A better way, is to convert the two signals from time domain to frequency domain. This can be achieved by FAST FOURIER TRANFORM. Once both the signals have been converted to frequency domain, they can simply be multiplied. Since Convolution in time domain is similar to multiplying in Frequency domain. Once both the signals have been multiplied, they can be converted back to time domain by Inverse Fourier Transform method. Thus achieving accurate results.