The answer lies in the difference between a ''Specifier'' and a ''Demonstrative''.
In English the words ''this'' and ''that'' can each function as specifiers or demonstratives.
The same goes for the Chinese words 这 and 那.
Specifiers occur as part of a noun phrase. This/That book is mine.
Demonstratives are used to point out an item. This/That is my book.
A simple heuristic is that demonstratives are followed by the verb ''to be'' or the ''copula''.
This is / 这是
Demonstrative. No counter used.
This book / 这本书
Specifier. Counter must be used.
For specifiers 这 and 那 must be followed by a counter or a number+counter.
This book. 这本书。
These three books. 这三本书。
这是什么？ What is this ? (This is what ?) Verb ''to be'' follows ''this'', therefore it is a demonstrative and doesn't require a counter.
这个是什么？ What is this thing/one/item ? Here the word ''this'' is part of the noun phrase ''this thing'', therefore it is a specifier and requires a counter.
那是我的自行车。 That is my bicycle.
那个是我的自行车。 That one is my bicycle.
Specifiers can be pronounced as nei and zhei, but demonstratives can not.
Specifier. You must say: 这(zhe)是什么？You can't say 这(zhei)是什么？
Demonstrative. You can say both: 这(zhe)个是什么？ or 这(zhei)个是什么？
这儿/那儿 and 这里/那里 are neither specifiers nor demonstratives and can't be pronounced as zhei or nei.
For the plural These/Those , demonstratives take 些 (这些/那些) while specifiers do not. Specifiers simpy specify using number+counter.
Demonstrative. These are mine. 这些是我的。
Specifier. These three are mine. 这三个是我的。
If the English starts with ''This/That + is'' or ''These/Those + are'' then no counter is required.