Sentence:  Many scholars suggest that ... the increase in the use of the "to"-infinitive in Middle English took place at the expense of the bare infinitive (i.e. an infinitive without the marker "to"). ... due to the loss of verbal inflections, it became difficult to distinguish the infinitival form from other verbal forms. As a result ... "to" began to function as a mere marker of the infinitive, losing its original ‘<WSD>purposive</WSD>' sense ... 
Meaning of the word marked with <WSD> tags:  Of a clause or conjunction: expressing purpose. (from 20th c.)