Is a relay needed?

Say I've got a motor that draws 5amps, can't I just use a switch rated at 10 or 15 amps instead of using a relay?

Do you think a relay is needed for a air horn solenoid, the solenoid would only be active for a few seconds?

in theory , yes you could use just a switch, but be wary of ratings. Your motor ,for example, may use 5amp nominal (after it is up to speed).

when power is first applied (to the motor), there is an "inrush" of current as the magnetic field builds in the motor coil. Also, the the current draw is higher to overcome inertial resistance (to get the motor turning). Sometimes these two factors can cause the intial startup current to be very high. Depending on the load on the motor, your 5 amp motor could take 7-12 amps at startup, thus overloading a typical switch.

Secondly, many switches "max" rating is only given for a short period of time (msecs.). usually the continuous current is a lot lower.

by using a relay, it is a safer more reliable way without having to calculate the specifics of each component (that's what engineering is for)

hope this helps