Why an Info-Object Should be Made Authorization Relevant in BW?

If you could read BW 365, that would give a comprehensive idea. For the time-being, info-object represents a column of table, from which query derives data. So, you need to restrict it, as required.

In other way, To get the required data you need to run a query. Each query will have some fields with restrictions. To restrict those values you need an analysis authorization. You can maintain the restrictions through Analysis Authorization only when the object is Auth relavent .