Why does the app icon not show up in Android if the MainActivity is not set as the LAUNCHER?

Question

When I install the application on my phone , if I do not put the Main Activity as the LAUNCHER, then the application does not show up on my Home Screen.

<application
    android:allowBackup="true"
    android:icon="@mipmap/icon"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <!-- This is the SplashScreen -->
    <activity android:name=".SplashScreen"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <action android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <activity
        android:name=".MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.default" />
        </intent-filter>

    </activity>

    <activity
        android:name=".SongList"
        android:label="@string/app_name" >
    </activity>
    <activity
        android:name=".DisplayLyrics"
        android:label="@string/app_name" >
    </activity>

</application>