Why not 'save' but 'dlmwrite' works?

I need to write 4 matrices into one file generated from one recursive structure one by one. The 'write' part could be done by using 'dlmwrite''-append' function. Like :

dlmwrite('outputC.txt',twoColumnResultC,'-append');

However, when I want to save those matrices as a .mat file by using 'save' function, like:

save ('C:\Users\SCU\Desktop\Qtree-work\output.mat','twoColumnResultC', '-append');

there goes

'Error using save |Unable to write file output: No such file or directory.'

I tried some solutions like changing the path, using 'fullfill', but still not work. The 'save' function works well independently, so I wonder if the logical structure of my code matters？ What is the difference between 'save' and 'dlmwrite'?

Thank you!