Architects need to 'know' the past built form because they are the ones given the responsibility to 'imagine' the future built form. Architects cannot imagine the 'future' unless they know where to go. They cannot know where to go unless they know where they came from.
But the moment is always 'now'. Now is the presence of the past and the future combined in a moment. Pause now and think of what you know about architecture, if you are given a task to design a house for example. You have two options: (1) blank mind and paper with 'nothing' on them but wonderful dreams and imaginations of what can be; and (2) a mind full of beautiful memory of childhood places and houses visited that you and others liked and wished you could design the same. Which approach would you choose?
You probably would not wish to choose option 1 by itself, because it may be difficult to imagine something without any 'reference' to make the 'house' realistic enough to be a 'home', to be liveble, to bring up children, to be attached to and be proud of. You need some conventions, some ideas that were proven to be successful. You also might not choose option 2 by itself because you feel the need to bring something more than 'just' memory of the past, to be more creative in problem solving that makes the house special.
The 'present', now, today, this year's design studio, is a 'passage' between the 'past' and the 'future', a 'moment' between memory and hope, a 'preserver' of collective knowledge and a 'generator' of new knowledge. The present is always 'in between' the past and the future. It cannot be between 'nothing' and the future or between the past and 'nowhere'. If this makes sense to you, then architectural history is an essential part to the formation of the architect you aspire to be.
History of Architecture
History is not the 'past'. History is 'accounts' of the past by people lived/live in the present. No one knows the past for sure except through two avenues: (1) written texts by people of the past who wrote about their own times; and (2) architecture itself as artefacts that inform us of the past. Every single book produced in our present time relies on the above two avenues to know the past. It follows that history of architecture is individual attempts to describe the past.
History of architecture, as individuals' accounts, is of two kinds that may overlap: (1) knowledge formed primarily from scholars' accounts of materials cultures based on art history and formal aspects of architecture such as building types and elements; and it relies heavily on styles description and detailed accounts of architectural parts, people are not included; and (2) knowledge of architecture as formal expression of different peoples and civilisations throughout history. These authors are with various intentions and background. In case of Islamic architecture, it was mostly written by western scholars, Orientalists and from specific interests such as archeology and art within cultural contexts.
Yet, for architects who realise the 'responsibility' to read the past for themselves and form their own interpretations and understanding, the two avenues stated above are available directly for their exploration. Not only their own past but the past of others are necessary to understanding. Civilizations do not exist in isolation; they are formed through exchange of collective knowledge. Traveling to see the past 'first-hand' is important for architects knowledge formation.