Running Maximum

Transho 
Apart from, Aliensx solution,
You can also achieve your result by using List.Accumulate function.. Below is the code

let
    Source = Table,
    ConvertingGroupingList = Table.Group( 
Source,{"Category"},{"AllRows",each _}
)[AllRows],
    Result = Table.Combine( 
List.Transform( 
ConvertingGroupingList,(x) =>
Table.FromColumns(
Table.ToColumns(x) &
{List.Select(
List.Accumulate(
x[Value],
{0},
(s,c) => s & {if List.Last(s) < c then c else List.Last(s) }
),each _ <>0 )}, Table.ColumnNames(Source) & {"Running Max"}
) ) )
in
    Result

Also, you can use List.FirstN function for achieveing the result.. Please note List.FirstN is very slow function and you may face performance issue if you are delaing with large dataset.You can use List.Buffer to speed up the performance but, List.Generate and List.Accumulate always preferable. Below is the List.FirstN code

let
    Source = Table,
    Result = Table.Combine( 
Table.Group(
 Table,{"Category"},{"AllRows", each 
let
IndexAdded = Table.AddIndexColumn(_,"Index",1,1),
MaxColumnAdded = Table.AddColumn(IndexAdded,"Running Max", each List.Max( List.FirstN(
List.Buffer(
IndexAdded[Value] ),[Index])) )
in 
MaxColumnAdded
}
)[AllRows]
)[[Date],[Category],[Value],[Running Max]]
in
    Result

Attaching pbix file for your reference where you will find both solution.

Hope this will help you.

Regards,

sanalytics