Why should the cloning vector be constructed first and then the expression vector be used? I guess it is because:
(1) to obtain a large number of DNA, although PCR is possible, there is an error rate in PCR amplification, but every time you do conventional PCR, you have to re-extract DNA and RNA every time. Moreover, the PCR reaction kit is not cheap, and it is worthwhile to prepare a large amount of DNA by PCR. Although the construction of cloning vector is more complicated (compared with PCR), the success rate is not very high and it needs to be screened, but once you successfully recombine and transform into Escherichia coli, it can be saved. When you want to use it, you can prepare a lot of DNA by shaking a bacterium.
② sequencing requirements. If you want to transfer expression, you must first determine whether the target fragment you amplify is what you want. Don't think that you can determine the length of your marker by designing a specific primer. This is just speculation, which is not rigorous in science. For safety reasons or if funds permit, the amplified gene fragments should be sequenced. Usually, the sequences sent for detection are sequences constructed on vectors, and cloning vectors usually carry sequencing primers. It has been confirmed that this is the sequence you want. To write a paper, you must give people real evidence.