Could you elaborate on that question? Are you asking the general usefulness that top would provide, or are you asking about "yarn top"? For now I'm assuming you meant "yarn top", using yarn top you can the current running applications and their usage statistics such as memory and cores used. It's a way to view whats going on more real time than the resource manager UI for example.
ex.
YARN top - 16:00:50, up 0d, 7:14, 1 active users, queue(s): root
NodeManager(s): 3 total, 3 active, 0 unhealthy, 0 decommissioned, 0 lost, 0 rebooted
Queue(s) Applications: 1 running, 3 submitted, 0 pending, 2 completed, 0 killed, 0 failed
Queue(s) Mem(GB): 0 available, 45 allocated, 155 pending, 0 reserved
Queue(s) VCores: 0 available, 90 allocated, 311 pending, 0 reserved
Queue(s) Containers: 90 allocated, 311 pending, 0 reserved
APPLICATIONID USER TYPE QUEUE PRIOR #CONT #RCONT VCORES RVCORES MEM RMEM VCORESECS MEMSECS %PROGR TIME NAME
application_1531989988564_0003 hdfs mapreduce default 0 90 0 90 0 45G 0G 357 189 5.00 00:00:00 QuasiMonteCarlo
