summarize

Cyriackpazhe , The Summarize function is used to create a summary table for the specified columns, but in the case it is not necessary for the filter condition to work correctly,

It can be applied directly also

CALCULATE([SalesAmount], FILTER(Sales, Sales[Color] = "Blue" || Sales[CategoryName] = "Accessories"))

It depends how you rewrite the query as to what result you will get. You could use FILTER on the sales table and get the same result, but filtering entire tables as opposed to just the columns you need is not best practice. Here the SUMMARIZE is reducing the number of columns passed to FILTER to just the necessary columns.

You could get the same result with

[Blue or accessories] =
CALCULATE (
    [Sales Amount],
    KEEPFILTERS ( 'Sales'[Color] = "blue" || 'Sales'[CategoryName] = "accessories" )
)

KEEPFILTERS is required to make sure that the measure doesn't overwrite the filter context coming from the matrix.