Why is solar eclipse so important for sunlight spectrum analysis? People wait for decades to catch the chance of a solar eclipse to observe the sun. 
Why cannot they do it every noon? 
 A: The corona is a sheath of exceedingly hot gas that surrounds the Sun. It's of interest to Solar scientists for all sorts of reasons, but normally it's impossible to make measurements of it because the light from the Sun's surface swamps any light coming from the corona. However in a total eclipse the Moon exactly masks out the surface of the Sun$^1$, and the light from the corona can then be seen clearly.
So the importance of a solar eclipse isn't that it allows analysis of the solar spectrum but that it allows analysis of the spectrum of the corona.
$^1$ not all eclipses exactly mask out the Sun. Depending on where in the Earth's orbit the eclipse happens the Moon can appear smaller than the Sun giving an annular eclipse, or bigger than the Sun so it masks some of the corona as well.
A: Having read John Rennie's answer you might think "What's the deal with masking the solar disk? It can easily be done by a traffic sign or any other circular obstacle!". The actual problem is not the brightness of the photosphere: it's the light scattered by the atmosphere that overwhelms the light from the solar corona.
Ever seen the Moon at daytime? Notice how faint it looks relative to the background sky (and remember how bright it looks at night in the same phase):

(photo taken by me)
And here it's half-lit, i.e. it's 90° away from the solar disk—one of the darkest directions in the daytime sky. Closer to the solar disk the light scattered by the atmosphere is much brighter due to the solar aureole.
And then, solar corona is much fainter than the Moon1. Hidden behind the atmosphere-scattered light and being exactly in the aureole area makes it guaranteed to be invisible at daytime, regardless of any earthly shadow throwers.
On the other hand, a total solar eclipse makes a huge shadow through the whole thickness of the atmosphere along the line Earth-Sun, resulting in the whole sky lowering brightness to that of twilight:

(image source)
Now these are the conditions where solar corona can easily be observed.

References:

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*NASA's Cosmos, figure titled "Corona Brightness and Electron Density"

