Sometimes when I am revising my writing I find I have jumbled up the tenses and need to pay attention to that as I edit. It can be hard to figure out what tense to use in reporting your results, especially when you are talking about your own results that are in front of you now, what you did (in the past) to get them and what others have done as reported in the literature, what people used to think and what is generally thought about now. All these types of things may be elements of your discussion and require a lot of attention to get right. If you don’t get it right it can completely change your meaning.
Just compare these two statements:
“microwave technology is an important tool…”
“microwave technology has been an important tool…”
They both say different things about how important microwave technology is now.
There are conventions to using tense in reporting which also helps the reader to follow what you are talking about. Both of these sites provide really good examples of what tense to use where. The one from Monash University is about general writing and literature reviews and the one from The University of Melbourne is about scientific reporting in particular.
Using Tenses in Scientific Writing, The University of Melbourne Student Services http://services.unimelb.edu.au/__data/assets/pdf_file/0009/471294/Using_tenses_in_scientific_writing_Update_051112.pdf
Use of tenses, Monash University Learning Support http://www.monash.edu.au/lls/llonline/writing/general/lit-reviews/3.2.xml