Why should I use an armrest?

[Frankdolin]

Since you asked why you "should", I'll just say IMHO you "shouldn't". Nothing against users of add-ons ,I'm just old school enough to think the mandolin is pretty perfect and needs only a good musician to be all it can be...

[AlanN]

Pickguard - nope
Armguard - nope
Toneguard - nope

Right Guard - yep

[UsuallyPickin]

My sweat damages the finish on my mandolin so I use an arm guard. I use a pickguard to protect the mandolin top from errant pick scratches. I use a Toneguard to increase the volume without pounding on my mandolin. Not that I don't pound ... occasionally.

[Hallmark498]

Pro for using armrest, no more holes in the sleeve of my shirts from the tailpiece.

[dhergert]

There are lots of good reasons for and against armrests (and tonegards, since they've been mentioned here too).

Basically if you're getting an armrest for comfort, get one that has gentle edges where your arm hits it. If you're getting one for tone or volume, make sure it doesn't rest on the soundboard of the instrument. If you're getting one to change your playing posture to avoid touching the bridge, make sure the armrest is high enough to do that. If you're getting one to preserve the finish, make sure the contact area of the armrest has nothing on it that will marr the finish under the armrest mount. If you're getting one because it looks cool, well, that's up to you.

Note that if you're getting a wooden armrest, there's nothing that says you cannot re-profile it to your preference.

Really, all of these benefits are based on your personal preference and how you execute using it.

In my experience ToneGard benefits, while brought up here in a similar mindset, are really less debatable. There are lots of other threads here on the Cafe' that discuss this. But again, personal preference overules all.

[soliver]

I've always felt like armrests are beneficial for more than just comfort and wear. I do like my "roll cage" (Tonegard) on the back of mine and feel like the armrest does the same for the top. It keeps my arm off the sound board and frees it up to move better than without. I honestly feel like the first time I put one on my mando, it did make a difference in the sound. I do recognize that that is subjective though much the same as folk's feelings about the tonegard.

[Bertram Henze]

Pro for using armrest, no more holes in the sleeve of my shirts from the tailpiece.

This is the exact reason for me to roll up my shirt sleeve when playing. I haven't had holes in my forearm yet - biological tissue seems to be more robust than fashion.