I am a newbie when it comes to using mne library, so I apologize if my question and future questions sound so simple, I want to understand better. I saw a post about volume source estimation with template MRI having this link EEG forward operator with a template MRI — MNE 1.6.1 documentation and reading few threads here I just realized the adult template MRI(fsaverage) helps in creating source space estimates without the use of FreeSurfer. My question is what is the use of the infant MRI surrogates?
That’s almost correct. fsaverage (or any surrogate MRI) lets you create source estimates without having an MRI scan for the subject (it’s not to avoid using FreeSurfer software). fsaverage has other uses too — for example, even when you do have individual MRI scans for each subject, it is common to “morph” them to align the sulci/gyri with fsaverage before doing statistical computations across subjects.
It’s much harder to get a clean MRI scan on a baby than on an adult, because they aren’t good at lying still (even when asleep). That is why (age-matched) surrogate MRIs are often used for infant MEG studies.