Learners learn grammatical structures in a sequence that reflects their complexity, rather than their use in communication, leading to many artificial contexts for practice, and perhaps an inability to transfer learning to real communication. Organising learning around a grammatical syllabus has been criticised because of this, but it is still the most common type of syllabus in published materials, mostly because it is the easiest type of syllabus to sequence.
Example
A grammatical syllabus may start with the present simple, then the present continuous, then the past simple, and so on. Learners are not usually exposed to more difficult structures than the ones they are learning.
In the classroom
Teachers may find it useful to blend a grammatical syllabus with other elements. For example, it may be suitable to teach verb times explicitly but structures such as the use of ‘would' in requests and advice in a functional framework instead.
Further links:
https://www.teachingenglish.org.uk/article/which-syllabus-traditional-holistic-syllabus
https://www.teachingenglish.org.uk/article/syllabus-writing
https://www.teachingenglish.org.uk/article/general-english-syllabus-design