The IoManager_DefaultMaximumReadAggregation setting controls how much data from individual read requests to the drive, can be combined into a single provider read request. It is expressed as bytes and defaults to 1 MB.
The reason for this setting is that we don't want to combine too many read requests due to the amount of time that it will take to download that data. For example, it may not be a good idea to combine 100 MB worth of disk reads into a single provider read. Downloading 100 MB may take a long time, and all of those individual disk read requests will have to wait for that to finish before making any progress.
That setting can be overridden by specific providers. For example, Local Disk and File Share default to 100 MB for that setting due to the expected higher throughput of those providers.