Why does we put Object obj as parameter inside compareTo method?

As you can see from the Java 8 documentation here: https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html

Comparable helps you compare two objects. Returning a negative value, zero, or positive value if the value is less than, equal to, or greater than the specified object.

Java knows how to compare things like Strings and Integers because they are built in classes within Java. But let's say you are writing a User class, a Post class (For a social network, for example), or a Photo class to store information about a photo, Java does not know how to determine whether two objects of one of these classes are equal to one another.

You, as the creator, may want to decide that a user is equal only if the usernames are identical, or maybe you allow similar usernames but unique registration emails. for your post class you may compare the text, but you may also want to compare the date and user who created a post, and on and on for different classes.

The Comparable interface, along with the CompareTo() method, allows you to specify under what scenarios two objects from a custom class are equal, and when one should go before another when sorting them.

Hope this helps :)