Sandboxed solutions, which are hosted in the SharePoint user code solution worker process (SPUCWorkerProcess.exe), run code that can only affect the site collection of the solution. Because sandboxed solutions do not run in the IIS worker process, neither the IIS application pool nor the IIS server must restart. Visual Studio attaches the debugger to the SPUCWorkerProcess process that the SPUserCodeV4 service in SharePoint automatically triggers and controls. It is not necessary for the SPUCWorkerProcess process to recycle to load the latest version of the solution.

Sandbox Solution is used to create / add some blocks of code for specific site, If we deploy solution as sandbox solution it will not affect SharePoint web application (i.e. it will never recycle application pool).So it will not affect other site collection.It mainly use to create/add content in SharePoint site to run in separate isolated environment(i.e. worker process).