On 21st April, Google updated the algorithm and made positive affect for SERPs of those websites which are mobile friendly. What is the effect of this algorithm/update on Desktop search?
There will be not any effect on desktop search. SERP will remain the same with desktop searches. Here is the Google FAQ http://googlewebmastercentral.blogspot.com/2015/04/faqs-april-21st-mobile-friendly.html
Its will only effect when you search anything on mobile. http://mobileoptimization.weebly.com/blog/-why-mobile-optimization-of-a-website-is-crucial-in-the-2015