Why is a FrameLayout used for fragments?

You can basically use RelativeLayout or LinearLayout it will still work,but the answer of your question is in FrameLayout's documentation :

FrameLayout is designed to block out an area on the screen to display a single item. Generally, FrameLayout should be used to hold a single child view, because it can be difficult to organize child views in a way that's scalable to different screen sizes without the children overlapping each other. You can, however, add multiple children to a FrameLayout and control their position within the FrameLayout by assigning gravity to each child, using the android:layout_gravity attribute.

You can read more about FrameLayout here : FrameLayout/Android Developers

Main purpose of frame layout is to block the area required to fit the largest child view. If you use a Frame Layout as Fragment Container you can ensure that you always have the space available to accommodate the largest fragment layout.

In some cases you may need to have more than 1 fragment on screen simultaneously in that case you should prefer Relative or Linear Layout.

Everything is based on user requirements and customization needed.

It is not necessary to use FrameLayout at all.

For example in below code there is no layout taken , and fragment itself only contains single imageview .

public class MyFragment extends Fragment implements OnClickListener{    String TAG="MyFragment";    Context c;    MyFragment(Context con){    c=con;    }    @Override    public void onCreate(Bundle savedInstanceState) {        // TODO Auto-generated method stub        super.onCreate(savedInstanceState);    }    @Override    public void onActivityCreated(Bundle savedInstanceState) {        // TODO Auto-generated method stub        super.onActivityCreated(savedInstanceState);    }    @Override    public View onCreateView(LayoutInflater inflater, ViewGroup container,            Bundle savedInstanceState) {        // TODO Auto-generated method stub        ImageView v=new ImageView(c);            //other imageview stuff        return v;    }    @Overrides    public void onClick(View v) {        // TODO Auto-generated method stub        Log.i(TAG, "onClcik");    }}