How does long vowels being written as cause Modern English to spell tense vowels this way?

Please see the sentences at the bottom, beside my red line?

1. How was "a long vowel being written as < VCe > so common"?

2. How does this appertain to the English convention "that this is a normal way to spell tense vowels"?

The letters <i> and <j> were in free variation: tiim tijm ‘time’, iuge juge ‘judge’ as were <u> and <v>: up vp ‘up’, euen even ‘even’. The letter <i> was considered allograph of <y>. One phonological change involving vowels also affected the relationship of vowels were neutralized to /ə/, language and writing. In Middle English unstressed and sometimes lost; the writing, however, retained the etymological vowel. Thus in writing today, one needs to know the etymological spelling of a word. For example, the first and third syllables of the words serene and divine were both unstressed and the vowel in these syllables became /ə/: Mid. Eng. /sə’re:nə/ and /də’vi:nə/. The final syllable is spelled <e> in both cases, but the first syllableis spelled differently in the two words, <se-, di->, reflecting the history of the words. Today, English speakers often stumble over pairs such as affect and effect; both words are normally pronounced /a’fekt/,but distinguished in spelling by the etymological vowel. In Late Middle English, unstressed <e> /ə/ was lost in word-final syllables: /də’vi:nə/ > /də’vi:n/, and /sə’re:nə/ > /sə’re:n/, but again the spelling did not change to reflect this change in pronunciation. The situation of a long vowel being written as <VCe> was so common that in Modern English the convention has arisen that this is a normal way to spell tense vowels: e.g., bite, made, rose, cute.

Henry Rogers, Writing Systems (2004), p 190.