it just measures Sound Pressure Level..
i dont see it as anything NEEDED for home stuff,but in the car..they use them all the time!
but be careful of claims of 170+ db..again,i (and i am sure many people who's jaw just dropped..) are aware there are different ways they measure db,(weighting..a..b..etc..) and there are ways to 'excite' certain mics with particular freq's,but a spectrum analyzer ,i think,is more of what someone wants/needs for home stuff..(well for people who care about the quality of reproduction,in,or out of the car)
anyway,to the point,
these things use a microphone to figure out what kind of sound pressure something is 'making' in a particular environment..
in our litigious world,people that work with jackhammers may sue someone if the spl is above some OSHA standard..
they use such a meter to determine how loud something is..measured in 'decibels..db' (Db?? i never cared! db is Db is dB..)