What are the benefits of using Krueger Flaps?
There are different types of Krueger flaps used, with increased efficiency coming at the cost of increased complexity.


The simple ones are basically flat panels that folds out from the wing lower surface into a very steep, deployed position and seals against the wing leading edge; though these are extremely simple, they don't offer much in the way of stall protection.
The next type has large folding 'bull' nose that offers increased performance at the cost of slightly increased complexity. This type of flaps is used in B727, B737, and inboard of B747, as shown in the figure below:




By Arpingstone - Photographed by Adrian Pingstone in August 2002 and released to the public domain., Public Domain, https://commons.wikimedia.org/w/index.php?curid=118310


The most complex type of Krueger flaps are the variable camber type, which has a flexible panel (which forms a slot at the trailing edge) and a bull nose. This provides a better $C_{L_{max}}$ performance compared to a slat, but gives poorer takeoff L/D performance compared to the slat. However, the main problem is that the mechanism is exceptionally complex, as can be seen in the figure below. As a result not many aircraft use these.




Image from aviation.stackexchange

This means that simpler Krueger flaps are used in the inboard with slats in the outboard region, so that the flow remains attached in the outboard regions at higher angles of attack compared to the inboard region.