What is function of filter in excel?

You would use the Filter function in the header row to filter out the relevant data/information provided in the filter drop down list.

Use an Advanced Filter in Excel to create a list of unique items, or to extract specific items to a different worksheet. You can also use complex criteria with an Advanced Filter, so it is useful when a simple AutoFilter can't do what you need.

its useful for :

1- Its Sort the data ( Assending or Decinding), by color, or select the number by a special spacification (Number equal to, greater than ...)  

2- enable you serach among in the column in which the filter applied on.

** you can make or remove it by ( CTRL + Shift + L )

filter is used to select specific data required.

It is done to make it easier to focus on specific information in a large database or table of data.

Temporarily hide unwanted data to use this function

Filter in Excel gives you the choice on how to arrange your data, whether it be chronological, by fill color or just by a mere text criteria. It helps you select various options so you can work easily on the task. Just make sure that when you filter, you do it for the whole data by removing the blank row space as it hinders filtering important/other/additional information underneath it. 

To view/get your desire results in large data

1. First click on the range of cells that you want to filter.

   Excel should automatically detect the current data range when applying the autofilter. However, if the range of cells that you want to filter has missing rows or columns of data, Excel will not select all the data, so you may need to manually select the range of cells that you want to apply the filter to.

2. 

   Select the Filter option from Data tab on the Excel ribbon (see right).

   You should now have the drop-down menus on each of your header cells, which can be used to select the rows to be displayed (see below).

The function of filter is to used to quick search of desired data.

Filter option helps you to segrigate a specific type of info you are looking for, from a vast data base. In other words filter option will show you the info you want to see or show and hides other data.