It's only necessary for older servers (e.g. Ubuntu 12.04 and 14.04) where people haven't done this already.
for example for precise the kernel ID is 7515 for the "DigitalOcean GrubLoader v0.2 (20160714) Ubuntu" kernel. If you do this be sure to double-check the id by listing the kernels using the API though. You can also do this with libcloud using 'ex_change_kernel' from the 'DigitalOcean_v2_NodeDriver'