Benefits of using WITH TABLOCK on an INSERT
I know of a few benefits but they're mostly situational.

Using TABLOCK will reduce concurrency but will immediately take a table lock on the target table. As long as you can guarantee that just one session will insert into the table this will avoid unnecessary row or page locks and will prevent lock escalation. After all, if you are inserting so much data that you'll get lock escalation anyway why not do it upfront?
If you are inserting into an empty page-compressed heap without TABLOCK all of the pages will have row compression instead of page compression:


The newly inserted row is page-compressed:

if new row goes to an existing page with page compression

if the new row is inserted through BULK INSERT with TABLOCK

if the new row is inserted through INSERT INTO ... (TABLOCK) SELECT FROM


Otherwise, the row is row-compressed.


In SQL Server 2016 the TABLOCK hint is required to get parallel insert into heaps, CCIs (Clustered Columnstore Indices), and local temp tables. There are lots of restrictions, some of which aren't documented. There can't be an IDENTITY column, the insert can't be done through an OUTPUT, etc.

See also The Data Loading Performance Guide