YOUR MORTGAGE IS NOT SET IN STONE FOR THE PERIOD OF ITS TERM. BETTER TERMS MAY BE AVAILABLE TO YOU RIGHT NOW.
You might want to switch lenders to get a better rate or different mortgage or renegotiate with your current lender. Another lender may have better rates or payment options. Refinancing essentially means paying off your mortgage and establishing a new mortgage with either a new lender or your current lender.
You are in effect registering a new mortgage with new appraisal, survey and legal costs. In theory you need to pay these over again although in practice lenders might offer incentives to help you switch. It is a competitive market after all.
There are many reasons why you might want to refinance, or increase, your existing mortgage — to consolidate non-mortgage debt, to finance improvements to your home, etc.