Add spacing into list of figures for each section

After each section add addtocontents{lof}{protectaddvspace{10pt}}.

This will insert a vertical space of 10pt between each set of figure titles in the LoF.

If you got bored with that you could change the section command to include doing the addtocontents.

If you need similar spaces in your LoT include addtocontents{lot}{protectaddvspace{10pt}}

% lofprob.tex  SE 564815

documentclass[12pt,a4paper]{article}

begin{document}
tableofcontents

listoffigures

section{First}
addtocontents{lof}{protectaddvspace{10pt}}

begin{figure}
centering
ILLSTRATION
caption{First}
end{figure}

begin{figure}
centering
ILLSTRATION
caption{Second}
end{figure}

section{Second}
addtocontents{lof}{protectaddvspace{10pt}}

begin{figure}
centering
ILLSTRATION
caption{First}
end{figure}

begin{figure}
centering
ILLSTRATION
caption{Second}
end{figure}

end{document}

Wrong class? That is what a book-like class usually does by default with figures of different chapters (not of sections). If you do not like the usual chapter style in your document, then scrbook or memoir class are good alternatives to have section-like chapters without headaches, and if start the chapters in a new page is also a problem (it should not be a problem in documents of some entity), it is easily solvable too:

documentclass{scrbook}
begin{document}
listoffigures
{letclearpagerelax chapter{One}}
begin{figure}[h]caption{aaa}end{figure}
begin{figure}[h]caption{bbb}end{figure}
{letclearpagerelax chapter{Two}}
begin{figure}[h]caption{ccc}end{figure}
begin{figure}[h]caption{ddd}end{figure}
end{document}