If I am a helicopter pilot and l know the GPS points of my starting point and ending point (assuming only two points) and l know my bearing, why would l need to know Magnetic magnetic Variance or what effect does magnetic Variance have?

For flying if your GPS goes down. at which point you would have to resort to using analog instrumentation i.e. a compass.

Magnetic Variations have no effect on your GPS. However calculating winds from True headings to Magnetic may come in handy if you are calculating just before take-off, or making a PIREP for none-GPS pilots.
Even your landing clearance with wind information expect to convert from True to Magnetic.

So you do not get lost. A GPS solves the problem though, MV is mainly used for VFR. Example: Going from point a to point b you have to go 21, where Im at the MV is -7.30 so my Magnetic Heading would have to be 14 or l would end up way off course.

It is not so much a factor when using newer navigation technologies, like GPS. However, if you were going to plot a course on a sectional chart and fly that course using a compass, then you would have to take magnetic variation into account. Reason why? You're compass is based on magnetic north while your charts are drawn with respect to true north and those two are not the same.

Variance as well as deviation need to be know so that you can convert your compass reading into a true direction. The Magnetic north pole is somewhere in Canada, not where the earth rotates. If you fly straight north you will end up at the Magnetic pole. This could be a problem say of you were starting out in San Diego and wanted to fly to Alaska. So a pilot uses the variance to correct the heading that they will fly to fly to the actual direction on the globe.

same reason a sailor does? keep from getting lost? magnetic VARIATION is different in all longitudes because the magnetic north pole is not in the same spot as the true north pole. so a magnetic heading must be corrected for variation as you travel accross longitudes or you will be headed the wrong way.

Magnetic Variation is only really a factor in deduced reckoning (dead reckoning) or pilotage navigation. Both of these involve calculating wind effects on the airplane to plot out a course across the ground.

Navigating by any electronic means (GPS,VOR,NDB) makes variation useless unless u r navigating by those means or when u r only navigating by compass. With GPS, u r flying to a ''spot which may or may not be an actual place. Either way, the GPS system already takes the variation into account.

With VOR u r flying a course to a physical navaid, so u depend on the course selected in the OBS or HS l & fly a heading to maintain that course, which depends more on wind speed & direction.

If u need help remembering it, remember: ''East is least, west is best'' You subtract easterly variation & add westerly variation, no matter which direction u r heading.

All these wonderful new navigation methods r wonderful as long as they keep working, which most of them usually do. But sometimes they quit on you, & u need to know the traditional navigation methods.

Magnetic variation is an essential part of navigating by magnetic compass. You still need to know what it means, in case the GPS goes Kaflooey.

On the other hand, if u never go anywhere you've never been before, u will always know where u r if u know how u got there.

--So you will know what you're doing, and know what you're talking about.

Not all helicopters have GPS.

Not all GPS units work.

Forecast winds aloft and wind near the airport surface reported in METARs and AWOS printouts are given with respect to True North.

The magnetic variation correction is made to get an idea of the wind direction with respect to airways, procedure tracks, and runways.

To prevent getting lost, it is just good airmanship