user49146: Why do we need level logic converter to have 5V or 3.3V?
Because in Arduino Uno/Mega (don't know about other boards), they already provide 5V and 3.3V for us to use.

user37622: Level logic converting for level shifting is different from powering a device.
Logic level is the state of a digital input/output. No voltage (or very little of it) is considered LOW.   Over the threshold voltage is HIGH. On a 3.3V system, if you set pin to HIGH, it has 3.3 V.  On a 5 V system, HIGH is 5 V. 
A digital input pin reads HIGH once the threshold voltage is surpassed. If the input pin is for 3.3 V and doesn't have 5 V tolerance, feeding 5 V as HIGH could destroy it.  That is why you need level converter from 5 V to 3.3 V.
If the input pin is for 5 V and the threshold voltage is higher then 3.3 V,  3.3 V input will not be read as HIGH.  That is when you need level converter from 3.3 V to 5 V.
You must consult the 3.3 V device datasheet and check if the input pins are 5 V tolerant.   You should also check the digital pin threshold voltage on a 5 V device in case you want to communicate with it using 3.3 V.

49146: <p>Why do we need level logic converter to have 5V or 3.3V?</p>

<p>Because in Arduino Uno/Mega (don't know about other boards), they already provide 5V and 3.3V for us to use.</p>
 37622: <p>Level logic converting for level shifting is different from powering a device.</p>

<p>Logic level is the state of a digital input/output. No voltage (or very little of it) is considered <code>LOW</code>.   Over the threshold voltage is <code>HIGH</code>. On a 3.3V system, if you set pin to <code>HIGH</code>, it has 3.3 V.  On a 5 V system, <code>HIGH</code> is 5 V. </p>

<p>A digital input pin reads <code>HIGH</code> once the threshold voltage is surpassed. If the input pin is for 3.3 V and doesn't have 5 V tolerance, feeding 5 V as <code>HIGH</code> could destroy it.  That is why you need level converter from 5 V to 3.3 V.</p>

<p>If the input pin is for 5 V and the threshold voltage is higher then 3.3 V,  3.3 V input will not be read as <code>HIGH</code>.  That is when you need level converter from 3.3 V to 5 V.</p>

<p>You must consult the 3.3 V device datasheet and check if the input pins are 5 V tolerant.   You should also check the digital pin threshold voltage on a 5 V device in case you want to communicate with it using 3.3 V.</p>
