Yes a decent letter of support helps greatly. There was a thread on the subject recently and have been others in the past with examples and advice.
the more info you give relevant the better also its your chance to answer or explain in detail any points which may help your case.
suporting letter is another name be use for letter. Make it one to two pages long but if certain situations need more information or explantion write more.
I had a supporting letter
Also made a power point file with pictures and explantion to show flat big enough for the two of us to suport the deeds and council ta xinfo.
I wrote two pages explaining my finances in detail and how i could support my wifey and how with her skills she would be an asset to the country and find employment(ie be ablw to pay tax but not be able to claim anything for two years).
im not suggesting writing all that out, but, i felt it necessary to make it more clearer for the eco and when writing all this info my wife was booked to have an interview and i wanted to make sure all the questions they might ask were all ready written down making it far easier fo my wife.
But as i have written before we were suddenly(not fun at the time due to it meaning evidence i was taking over in time for the interview had to be fedexed at great cost grr) one of the first couples to just have to send in info, and if satsfactory not require an interview, which was the case.
take some time with your suporting letter its worth it.