Use of a filter in analytical tools is used to define the ways the data will be reported. Filters are applied prior to report and manipulated data will only be viewable afterwards. Most frequently filters are used to exclude the various fraudulent traffic to the website in order to clean the data and observe only the real visitors. Examples of fraudulent traffic are bots and spam traffic.
The filters in Google Analytics are applied at view level. Once enabled the filters will function and alter the data. Manipulated data can no longer be viewed in their original form, so filters should be deployed carefully in order to avoid data loss.